Recent changes in the opacity tables, and the discovery of many new Cepheids in other galaxies indicates a need to review the theoretical models of Cepheids in other galaxies, as well as in the Milky Way. The pulsational characterisitics will be examined, as well as the likely pulsational modes for Cepheids in various galaxies.

"Fourier Coefficients of OGLE Variables. I. Parameters for the Baade's Window, MM5-A, MM5-B, MM7-A, and MM7-B Field Variables", MORGAN, SIOBAHN M., SIMET, MELANIE, BARGENQUAST, STEPHANIE B., in Acta Astronomica, vol. 48, p. 331-339, 1998

Fourier coefficients for the variable stars in the OGLE survey of the galactic center were derived. In most cases the classification of the variable type is confirmed but there are some cases where the classification is brought into question. The Fourier coefficients for all variables including RR Lyrae, Delta Scuti and SX Phe stars, as well as an Anomolous Cepheid are presented. In general the distribution of the Fourier coefficients for the different types of variable stars in this study lie within the observed ranges of other studies.

"Fourier Coefficients of OGLE Variables. II. RR Lyraes" MORGAN, SIOBAHN M., SIMET, MELANIE, BARGENQUAST, STEPHANIE B., in Acta Astronomica, vol. 48, p. 341-353, 1998

Fourier coefficients for RR Lyraes in the OGLE survey are compared to the coefficients of field and cluster RR Lyraes. Based upon the location of the Bailey type ab and c stars for a range of metallicity, it appears that the vast majority of the OGLE variables have a [Fe/H] near -1.1. Using both empirical and theoretical relations for the Fourier coefficients, stellar characteristics are found to be similar to those derived in other RR Lyrae studies. There are also some stars in the OGLE sample which appear to be of a slightly different metal abundance, as well as some which are members of the Sagittarius dwarf galaxy. These latter stars do show a slight metal deficiency relative to most of the OGLE stars. The OGLE variables give a distance to the Galactic bulge of approximately 8 kpc, and a distance to the Sagittarius dwarf galaxy of about 25 kpc.

"Fourier Coefficients of OGLE Variables. III. delta Scuti Stars" MORGAN, SIOBAHN M., SIMET, MELANIE, BARGENQUAST, STEPHANIE B., in Acta Astronomica, vol. 48, p. 509-518, 1998

Fourier coefficients for delta Scuti from the OGLE dark matter survey of the Galactic bulge are compared to the coefficients of similar stars, mainly High Amplitude delta Scuti and SX Phe stars. While in previous studies there appears to be a division amongst the delta Scuti stars based upon the R₂₁ parameter, such a division is not apparent in the OGLE stars. There does though appear to be a separation of the delta Scuti stars based upon the phi₃₁ variable. This separation is not likely due to a metal variation and its cause is not known. Values for the extinction towards the Galactic bulge can be used to estimate a distance of approximately 7.6 kpc for the delta Scuti stars. One variable does appear to be located beyond the Galactic bulge at a distance of approximately 21 kpc.

"Beat cepheid period ratios from OPAL opacities" MORGAN, SIOBAHN M., WELCH, DOUGLAS L.,Astronomical Journal, vol. 114,Sept. 1997, p. 1183

The discovery of a large number of beat Cepheids in the Large Magellanic Cloud in the MACHO survey, provides an opportunity to compare the characteristics of such Cepheids over a range of metallicities. We produced a large grid of linear nonadiabatic pulsation models using the OPAL opacity tables and with compositions corresponding to those of the Milky Way, and the Large and Small Magellanic Clouds. Using the relationship between the period ratio and the main pulsation period, we are able to define a range of models which correspond to the observed beat Cepheids, and thereby constrain the physical characteristics of the LMC beat Cepheids. We are also able to make some predictions about the nature of the yet-to-be-discovered SMC beat Cepheids.

"The dynamical mass of the beat cepheid Y carinae and stellar opacities", BOHM-VITENSE, E., EVANS, N. R., CARPENTER, K., MORGAN, S., BECK-WINCHATZ, B., ROBINSON, R.,Astronomical Journal, vol. 114,Sept. 1997, p. 1176

The mass-luminosity relation for Cepheids depends on the degree of excess mixing in their main sequence progenitors. The mass determination for Cepheids with their known luminosities therefore determines the degree of excess mixing in massive main sequence stars. We have determined the dynamical mass of several Cepheids with blue companions. Here we discuss the beat Cepheid Y Carinae. By means of HST, GHRS spectra we measured the radial velocity of the B9.5 V companion Y Car B at phases near minimum and maximum orbital radial velocities. The orbital velocity amplitude ratio between the Cepheid and the hot companion and thereby the mass ratio comes out to be 1.51 0.5, leading to a mass of 3.8 1 1.2 solar masses. Taking the 3.8 solar masses at face value this indicates excess mixing corresponding to convective overshoot by about 1 pressure scale height, (see Bertelli et al.1986), but the error limits are too large for a firm conclusion. The beat masses, determined from the period ratios for the beat Cepheids, have puzzled astronomers for a long time (see Cox 1980) because they came out around 1 to 2 solar masses, when the Cox-Tabor opacities were used for the model calculations. Moskalik et al.(1992) showed that beat masses around 4 to 5 solar masses can now be derived, if the new OPAL opacities are used instead. With the determination of the dynamical mass for Y Car A we can now check the validity of the OPAL opacities more quantitatively than was possible before. Using the model calculations incorporating the OPAL opacities, and the observed effective temperature of Y Carinae we determine for its beat mass a possible range of 3.75 < M/Ms < 4.0, in very good agreement with the dynamical mass. This supports the validity of the OPAL opacities, and also supports the conclusion about the high degree of excess mixing in the main sequence progenitor of Y Carinae A.

"A New Method to recognize s-Cepheids", MORGAN, SIOBAHN M.Astronomical Journal, vol. 109,March 1995, p. 1263

A new method of delineating sinusoidal or s-Cepheids is presented. The method uses the values of (the mean intensity), V (the average magnitude) and V_mean (the value of the mean magnitude). Fourier coefficient data from galactic Cepheids is used to derive these terms in the V band and the differences between the various terms show systematic trends with increasing period. The Cepheids can be easily grouped into 3 divisions - short period s-Cepheids, intermediate period Cepheids (P < 9 days) and long period Cepheids (P > 9 days). Cepheids previously designated as s-Cepheids by others are compared to those found using the method outlined here. The method is also applied to Cepheids in the Small Magellanic Cloud to examine its suitability as a pulsation mode discriminator.

"An opacity mechanism for the pulsations of OB stars", COX, ARTHUR N.; MORGAN, SIOBAHN M.; ROGERS, FORREST J.; IGLESIAS, CARLOS A. Astrophysical Journal, vol. 393, no. 1, July 1, 1992, p. 272-277.

It is proposed that the sudden appearance of a tremendous number of same-shell transition iron lines, as the temperature rises above 100,000 K, gives a high sensitivity of the opacity to temperature at the very low densities found in OB giants. This produces kappa effect pulsations. The reason not all B stars pulsate could be that a slight primordial deficit in the iron abundance in the surface layer reduces the opacity and its sensitivity to temperature and reduces the kappa effect driving. A slight amount of iron concentration by radiative levitation could cause a star to pulsate even if it did not originally have enough primordial iron to cause this mechanism to operate. Then any slow slight mixing caused by the unstable shearing nonradial pulsations could restabilize the pulsations as actually observed in Alpha Vir and Beta CMa. Rapid levitation and mixing for the very luminous B stars with their very low density envelopes could even explain luminous blue variables that pulsate only a few cycles before they stabilize again. Large amplitude pulsations like those seen in BW Vul would indicate a somewhat large primordial iron abundance compared to all other B stars. Multimode behavior is theoretically expected for this pulsation mechanism, and for some B stars only a few nonradial modes (possibly selected by rotation) may survive to observable amplitudes.

"Pulsars as spiral arm tracers" MORGAN, SIOBAHN Astronomical Society of the Pacific, Publications, vol. 102, Jan. 1990, p. 102-106.

Pulsars are used to trace the spiral arms of the Galaxy. Present pulsar characteristics are used to divide them into two pulsar types as defined by Huang (1987). The ages of Type II pulsars and the galactic rotation curve are used to determine their position at birth. The spiral arm pattern velocity is used to reconstruct the arms from which the individual Type II pulsars originated. It is found that the best fit to the data requires spiral arm segments with a pitch angle of 14.5 deg.

"Pulsations of B star models by an opacity mechanism" COX, ARTHUR N.; MORGAN, SIOBAHN M. Presented at the Confrontation Between Stellar Pulsation and Evolution, Bologna, Italy, 28-31 May 1990

The pulsation mechanism for B stars has been sought for 30 years. No proposed radial or nonradial mechanism, either deeply seated or in the surface layers, has been successful in explaining all the observational details. Perhaps the missing piece in the puzzle is the opacity of the stellar material. Many times the first author has tried to make unconventional surface compositions give instability, but none were ever found. We now propose that the sudden appearance of a tremendous number of iron lines, as the temperature rises above about 150,000 K, gives a high sensitivity of the opacity to temperature at the very low densities found in these blue giants. Opacities need to increase quickly to a factor of three or more above the Cox-Tabor (1976) values in the range around 200,000 K. These increases are the same needed to decrease theoretical period ratios of double-mode Cepheids and (delta) Scuti variables to agree better with observations for conventional yellow giant masses. The reason why not all B stars pulsate is that a slight primordial deficit in the iron abundance in the surface layer (1 times 10 to the minus 6 power of the mass) can reduce the opacity and its sensitivity to temperature. A slight amount of iron concentration by radiative levitation could make a star pulsate even if it did not originally have enough primordial iron to cause this opacity mechanism to operate. Then any slow slight mixing caused by the unstable nonradial pulsations could restabilize the pulsations as actually observed in (alpha) Vir and (beta) CMa. Rapid levitation and mixing for the very luminous B stars with their very low density envelopes could even explain the puzzling luminous blue variables with this standard (kappa) mechanism. Large amplitude pulsations like those seen in BW Vul would indicate a somewhat larger iron abundance compared to all other B stars.

"Cepheid masses for models with enhanced opacities" MORGAN, SIOBAHN M.; COX, ARTHUR N. Presented at the Confrontation Between Stellar Pulsation and Evolution, Bologna, Italy, 28-31 May 1990

Cepheid models with enhanced opacity are constructed and used to determine pulsation constants, Q(sub 0). These are compared to models without an enhanced opacity. Methods of determining Cepheid masses are investigated using the different models and are compared to results obtained by Gieren. The methods to determine the pulsation and Wesselink masses differ significantly, due to the use of a Q(sub 0) that varies with mass, radius and luminosity.

"Problems with the Baade-Wesselink method" BOHM-VITENSE, E.; GARNAVICH, P.; LAWLER, M.; MENA-WERTH, J.; MORGAN, S. Astrophysical Journal, vol. 343, Aug. 1, 1989, p. 343-351.

The discrepancy noted in radii obtained by the Baade-Wesselink method when different colors are used to determine the effective temperatures is explored. The discrepancy is found to be due to an inconsistency in the applied temperature-color calibrations. The assumption of the maximum likelihood method that beta (the effective temperature + 0.1 times the bolometric correction) is a linear function of the color is valid for the B-V and V-I colors, but not for the V-R colors. It is suggested that the errors introduced by the nonlinearity in the relation between beta and the V-R colors will produce radii which are too large. The radii derived from the V-B colors appear to be too small.

"The self-enrichment of globular clusters" MORGAN, SIOBAHN; LAKE, GEORGE Astrophysical Journal, vol. 339, April 1, 1989, p. 171-177.

It is shown that protoglobular clusters of primordial gas can confine the supernovae needed to enrich themselves. The required protocluster cloud masses and structural parameters are the same as those currently observed for the clusters. Two causal scenarios for star formation are examined to calculate the initial enrichment of primordial clouds. In the 'Christmas tree' scheme, the maximum final (Fe/H) is about 0.1. Since the time scale for formation and evolution of massive stars at the center of a cluster is nearly an order of magnitude less than the collapse time of the cluster, every globular cluster may have to survive a suprernova detonation. If this is the case, the minimum mass of a globular cluster is about 10 to the 4.6th solar mass.

"Effects of SN 1987A on the interstellar medium around it" BOHM-VITENSE, ERIKA; BOGGS, DON; MORGAN, SIOBAHN Astronomical Journal, vol. 97, Jan. 1989, p. 131-138.

It is shown that the interstellar spectrum from SN 1987A in the LMC shows a rather strong component in C IV and Si IV, and possibly a weak component in N V, at a velocity of about 300 + or - 10 Km/s. In the spectra of the neighboring stars HD38268 and R123, a component at this velocity is only seen in low-ionization lines, not in the C IV, or N V lines. The possibility is studied that SN 1987A ionized a measurable fraction of this 300 Km/s cloud within less than one day since the Si IV, C IV, and possibly, N V interstellar lines are seen only about one day after the first light from the supernova explosion was detected. This may be the case, but only if the supernova temperature at outburst was larger than 50,000 K, depending on the density assumed for the interstellar cloud. Also, H and He must have been preionized once.

"ISM chemical abundances along the line of sight to SN 1987A" MORGAN, SIOBAHN; BOEHM-VITENSE, ERIKA In ESA, Proceedings of the Celebratory Symposium on a Decade of UV Astronomy with the IUE Satellite, Volume 2 p 207-209 (SEE N89-16535 08-89)

The IUE high resolution spectra of SN 1987A were studied. Absorption lines of 2 clouds with velocities of 130 km/sec and 180 km/sec (heliocentric) are seen in addition to the local galactic gas and the Large Magellanic Cloud (LMC) gas. Curve of growth analysis shows that these clouds may be depleted in heavy elements when compared to the local ISM and the LMC ISM. There is no indication of depletion due to grain formation. There seems to be no way to determine whether the clouds are part of the galactic halo or part of the LMC's halo.

"ISM chemical abundances in two intermediate-velocity clouds in the line of sight to SN 1987A" MORGAN, SIOBAHN; BOHM-VITENSE, ERIKA Astronomical Journal, vol. 96, Oct. 1988, p. 1373-1382.

The earliest IUE high-resolution spectra of SN 1987A have been studied and reveal the presence of several clouds in the line of sight to the LMC. In particular, there are two clouds with radial velocities of about 130 km/s and about 180 km/s. These clouds' velocities are between those of Galactic clouds at 0-80 km/s and those of LMC gas at about 270 km/s. Chemical-abundance determinations may help to determine the origin and location of these clouds. Curve-of-growth analysis and 21-cm observations show that they may be underabundant in heavy elements by about a factor of 2 as compared to solar abundances. No depletion indicative of grain formation can be seen.

"Cepheid envelope models" MORGAN, SIOBAHN M. Ph.D. Thesis Univ. of Washington, Seattle, 1991

Models of Cepheid envelopes are constructed to examine their characteristics and compare them with observations. The models are initially in hydrostatic equilibrium and the mass luminosity relations of different stellar evolution calculations can be assigned groups of these models. A method for an explicit hydrodynamics and implicit radiative transfer solution to the pulsational variation of the Cepheid models is described. This method is applied to the same series of models to obtain light and velocity curves. In most cases the pulsational characteristics of the models are similar to the results obtained by the Linear Non-Adiabatic analysis. Thus, the models that correspond to the highest luminosity stellar evolution calculations give the best fit to empirical Cepheid relations. The same method of analysis described is also applied to a series of models that have opacities increased along the lines suggested by recent opacity calculations. In most cases, the model characteristics are similar to those for the models described. It is found that the models that best fit the empirical relations of Cepheids are those that correspond to the high luminosity stellar evolution calculations.

"Pulsation Modes of SMC Cepheids", PARKER, W. J.; MORGAN, S. M. Bull. American Astron. Soc., 184, #55.07

The pulsational characteristics of Small Magellanic Cloud Cepheids is examined and compared to the results obtained from linear non-adiabatic models. The possible existence of up to 3 different pulsation modes (fundamental, first and second overtone) is studied and the characteristics, in particular the masses, of the Cepheids which are pulsating under these different modes is presented. The limits of the Cepheid Instability Strip derived from the models for the different modes is also presented and compared with the observed Instability Strip.

"The Theoretical Instability Strip for Luminous Blue Variables", SOUKUP, MICHAEL S.; COX, ARTHUR N.; GUZIK, JOYCE A.; MORGAN, SIOBAHN M., Bull. American Astron. Soc., 184, #31.01

Luminous blue variables (LBVs) occur in a small blue region of the H-R diagram appropriate for the most massive stars in galaxies. LBVs seem to appear only when mass loss before or during blue loops has removed the normal hydrogen-rich envelope to reveal hydrogen burning products like extra helium and nitrogen. Livermore OPAL opacities allow the observed microvariations now to be interpreted as nonradial g-mode pulsations of low angular degree. If the many unstable modes concentrate their energy into only one or a few modes, outbursts and extensive mass loss may occur as amplitudes rapidly grow. Then pulsations stop since the mass in the pulsation driving region decreases temporarily. Theoretical predictions require that convection be suppressed so that the standard kappa effect from iron line-enhanced opacities can give enough pulsation excitation to make these stars variable. A composition gradient at the surface, exposed by extensive mass loss during previous evolution, can accomplish this suppression. The blue and red edges of the pulsation instability strip could be due to the establishment of this steep enough mu gradient at the surface. However, too much mass loss, so that iron line opacity enhancement is overwhelmed by excessive helium, can prevent pulsations. In this study we find the more conventional explanation for the blue and red edges is likely. When our models are too hot and blueward of the blue edge, the driving layer at about 200,000K is too shallow to involve enough stellar material to overcome the deeper damping. When a model is too cool and redward of the red edge, driving is too deep and almost adiabatic on the pulsation time scale, giving again too little excitation. These blue and red edges depend on the stellar mass, luminosity, population type and the internal structure the evolution produces. We find that the theoretical instability strip edges between log T_e of 4.4 and 4.1 are close to that observed for Population I stars. The strip is narrower for LBVs in lower metallicity galaxies.

"Correlation of Martian Polygonal Pattern Sizes and Shapes with respect to Latitude", KURTZ, T.; WEBER, J.; MORGAN, S. M.; WALTERS, J. C., Bull. American Astron. Soc., 184, #04.06

Martian permafrost features are found in the northern latitudes of the planet. The distribution of patterned ground was studied and correlated to latitude. The results of this study indicate an increase in the sizes of polygonal shapes with latitude. The vertices, sorting and order of the polygonal shapes was also examined. This correlation in size corresponds to that which is seen in terrestrial polygons.

"A New Method for the Definition of s-Cepheids", MORGAN, S. M., Bull. American Astron. Soc., 184, #55.06

A new method of delineating sinusoidal or s-Cepheids is presented. The method uses the values of < V > (the average intensity), /line{V} (the average magnitude), and V_{median} (the value of the median magnitude). Fourier coefficient data from galactic Cepheids is used to derive these terms in the V band and the differences between the various terms show systematic trends with increasing period. The Cepheids can be easily grouped into 3 divisions - short period s-Cepheids, intermediate period Cepheids (P<9 days), and long period Cepheids (P>9 days). The Hertzsprung progression has vitually no effect on the difference values over the various period ranges. Cepheids previously designated as s-Cepheids by others are compared to those found using the method outlined. The method is also applied to Cepheids in the Small Magellanic Cloud to examine its suitability as a pulsation mode discriminator.

"Cepheid Envelope Models", MORGAN, SIOBAHN M., Publications of the Astronomical Society of the Pacific, v.105, p.123

Models of Cepheid envelopes are constructed to examine their characteristics and compare them with observations. The models are initially in hydrostatis equilibrium and the mass-luminosity relations of different stellar-evolution calcuations can be assigned to groups of these models. Linear nonadiabatic pulsation analysis of the models provides an estimate of the period and expected pulsation mode. The pulsation mode depends on the period, with first-overtone pulsators expected for models with periods less than 10 days and fundamental-mode pulsators for longer periods. We find that those models with the higher luminosity for a given mass have characteristics similar to most binary and dynamical masses, and that empirical period-radius period-luminsotiy-temperature, and period-luminosity relations are found to correspond closest to the high-luminosity stellar-evolution calculations.

A method for an explicit hydrodynamics and implicit radiative-transfer solution to the pulsational variation of the Cepheid models is described. This method is applied to the same series of models to obtain light and velocity curves. In most cases the pulsation characteristics of the models are similar to the results obtianed by the linear nonadiabatic analysis. Thus, the models tha tcorrespond to the highest-lumnosity stellar-evolution calculations give the best fit to empirical Cepheid relations. The phase lag between the minimum radius and the luminosity maximum is found to be dependent on temperature. The Fourier coefficients for the light and velocity curves are determined, and it is found that the light-curve coefficients have a greater scatter relative to the coefficients obtained from observed Cepheid light curves, while the theoretical velocity-curve coefficients are quite similar to those obtained from Cepheid observations. For those models that have bumps on the light curves, the bump Cepheid period-radius relation for these stars is very similar to that obtained from observed bump Cepheids.

The same method of analysis described is also applied to a series of models that have opacities increased along the lines suggested by recent opacity calculations. In most cases the model characteristics are similar to those for the models described above. It is found that the models that best fit the empirical relations of Cepheids are those that correspond to the high-luminosity stellar-evolution calculations.

"Predicted Nonradial Pulsations of Luminous Blue Variables", COX, ARTHUR N.; MORGAN, SIOBAHN M.; SOUKUP, MICHAEL S.; GUZIK, JOYCE A., Bull. American Astron. Soc., 183, #101.02

Luminous blue variables (LBVs) are known to vary on many-decade timescales like eta Car and P Cyg, on many-year timescales as their usual observed outbursts, and with many-day period microvariations that could be stellar pulsations. These pulsations, observed only when the stars are not in outburst, may be intrinsic, just as for the less luminous beta Cephei and related variables. This investigation studies whether the new Livermore OPAL opacities can allow theoretical predictions of nonradial g-modes. A model with initial mass of 50 M_sun and current mass of 38.85 M_sun at a luminosity of 5.36x10(5) L_sun and surface effective temperature of 24,430K has unstable few day nonradial modes near g_5 with degree l=1. An approximate allowance for the effects of time-dependent convection in the driving layers between 10(-6) and 10(-5) of the mass into the model can nullify the radiation kappa effect and stabilize predicted pulsations. Thus it seems necessary to suppress convection so that the driving is not reduced to a value less than the radiation damping occurring in deeper lying layers. A likely convection suppression cause is a composition mu gradient produced by rapid helium diffusive settling in layers that originally were not convective. When the temperature gradient becomes superadiabatic in the gradually deeping layers as mass loss proceeds off the surface, a helium mu gradient can prevent convection and allow pulsations. Possibly outbursts are caused by pulsations driven so strongly that they get out of hand. They then remove any inhomogeneous composition layers. As the outburst recovers, helium settling below any new surface convection zone can reestablish the mu gradient that can again suppress convection when the deepening convection zone arrives there. Cycles of pulsations and an outburst followed by no pulsations can occur in this very thin surface layer as the star evolves to cooler F star regions with greatly deepening convection and no pulsational instabilities. The hundreds of days periods observed are probably the result of beating between our predicted adjacent few day period g-modes that could be observed individually.

"Fourier Coefficients of OGLE Variables", Morgan, S.; Simet, M.; Bargenquast, S.; Dickerson, C, in The Impact of Large-Scale Surveys on Pulsating Star Research, ASP Conference Series, Vol. 203; also IAU Colloquium #176. Edited by L.Szabados and D. Kurtz, 2000, p.275

Fourier coefficients for the variable stars in the OGLE survey of the Galactic Center and several globular clusters are presented. The characteristics of the RR Lyrae, d Scuti and SX Phe stars are presented and a comparison to those observed in other studies is made. In some cases the coefficients of the stars can be used to determine distances and the physical characteristics of the variables.

"Fourier Coefficients of OGLE Variables IV. RR Lyrae Stars in omega Cen and towards 47 Tuc", Morgan, S.; Dickerson, C. in Acta Astronomica, vol. 50, p. 249-268, 2000

Fourier coeficients for RR Lyr stars found by the OGLE survey in omega Cen and in the direction of 47 Tuc are presented. A total of 180 light curves were examined. Physical parameters for the stars including mass, luminosity, effective temperature and metallicity were derived using theoretical relations based upon the Fourier coefficients. The well known metallicity variation is seen in the omega Cen RR Lyr variables. The distance modulus to omega Cen is estimated to be =13.73 +/- 0.07 mag, while the RR Lyr stars seen in the direction of 47 Tuc are located in the Small Magellanic Cloud, and have an estimated distance modulus of =18.78 +/- 0.08 mag.

"Masses for Galactic Beat Cepheids", D'Cruz, N. L., Morgan, S. M., Bohm-Vitense, E., in Astronomical Journal, vol. 120, p. 990-997, 2000

Accurate mass determinations for Cepheids may be used to determine the degree of excess mixing in the interiors of their main-sequence progenitors: the larger the excess mixing, the larger the luminosity of the Cepheid of a given mass, or the smaller the mass of a Cepheid with given luminosity. Dynamical masses determined recently for a few Cepheid binaries indicate excess mixing somewhat stronger than that corresponding to the convective overshoot models by Schaller et al. Beat Cepheids can be used similarly to test main-sequence mixing in stellar interiors. The period ratios for beat Cepheids depond on luminosity, T_eff, heavy element abundance, and mass. By comparing pulsational models and the observationally derived luminosity, T_eff, metallicities and period ratios it is possible to obtain masses for these stars, the so-called beat masses. With the old opacities masses much smaller than the evolutionary masses were obtained. With the new OPAL opacities a beat mass close to the dynamical mass was obtained for the binary beat Cepheid Y Carinae, showing that it is now possible to obtain reliable beat masses. In this paper, we determine beat masses for seven Galactic beat Cepheids for which photometric and spectroscopic data are available. We find an average mass around 4.2 +/- 0.3 M_solar for these stars, though the actual error limits for each star may be larger mainly because of uncertainties in E(B-V) and the heavy element abundances. (As derived spectroscopically, beat Cepheids are in general metal-poor, with -0.4 < [Fe/H] < 0.0). The relation between the derived beat masses and the luminosities again indicates excess mixing that is somewhat larger than that corresponding to the models by Schaller et al.