Masses for Galactic beat Cepheids

Accurate mass determinations for Cepheids may be used to determine the degr ee of excess mixing in the interiors of their main-sequence progenitors: th e larger the excess mixing, the larger the luminosity of the Cepheid of a g iven mass, or the smaller the mass of a Cepheid with given luminosity. Dyna mical masses determined recently for a few Cepheid binaries indicate excess mixing somewhat stronger than that corresponding to the convective oversho ot models by Schaller et al. Beat Cepheids can be used similarly to test ma in-sequence mixing in stellar interiors. The period ratios for beat Cepheid s depend on luminosity, T-eff, heavy element abundance, and mass. By compar ing pulsational models and the observationally derived luminosity, T-eff, m etallicities, and period ratios it is possible to obtain masses for these s tars, 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 Ceph eid Y Carinae, showing that it is now possible to obtain reliable beat mass es. In this paper, we determine beat masses for seven Galactic beat Cepheid s for which photometric and spectroscopic data are available. We find an av erage mass around 4.2 +/- 0.3 M. for these stars, though the actual error l imits for each star may be larger mainly because of uncertainties in E(B-V) and the heavy element abundances. (As derived spectroscopically, beat Ceph eids are in general metal-poor, with -0.4 less than or similar to [Fe/H] le ss than or similar to 0.0). The relation between the derived beat masses an d the luminosities again indicates excess mixing that is somewhat larger th an that corresponding to the models by Schaller et al.