CLASSICAL CEPHEIDS WITH TERMINAL AGE MAIN-SEQUENCE COMPANIONS - CONSTRAINTS ON EVOLUTION

Eight binaries containing a Cepheid and a hot companion evolved beyond the ZAMS are studied using IUE spectra. The spectral types of these T AMS (terminal age main-sequence) companions are determined from the sp ectra and absolute magnitudes are determined from the magnitude differ ence between the two stars and the absolute magnitude of the Cepheid. The systems with Cepheids with periods less than 6 days (BP Cir, AX Ci r, and V659 Cen) match isochrones with no core convective overshoot. T he 12 day Cepheid SY Nor is better matched with an isochrone with mild overshoot. The 11 day Cepheid SV Per and the 16 day Cepheid RW Cam do not match any available isochrone (with any overshoot value) because the companions are cool enough to be at the very end of the main-seque nce phase but the magnitude difference between the two stars is too la rge for this scenario. KN Cen and AW Per may be similar systems. These unusually cool companions are common (up to half the sample) and dist ributed throughout the Cepheid mass range. Rotation could explain thes e companions. In this case, use of the main-sequence width to determin e the importance of convective overshoot would produce an overestimate of convective overshoot.