Classical Cepheids with Terminal Age Main-Sequence Companions: Constraints on Evolution

Abstract

Eight binaries containing a Cepheid and a hot companion evolved beyond the ZAMS are studied using IUE spectra. The spectral types of these TAMS (terminal age main-sequence) companions are determined from the spectra and absolute magnitudes are determined from the magnitude difference between the two stars and the absolute magnitude of the Cepheid. The systems with Cepheids with periods less than 6 days (BP Cir, AX Cir, and V659 Cen) match isochrones with no core convective overshoot. The 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-sequence phase but the magnitude difference between the two stars is too large for this scenarior. KN Cen and AW Per may be similar systems. These unusually cool companions are common (up to half the sample) and distributed throughout the Cepheid mass range. Rotation could explain these companions. In this case, use of the main-sequence width to determine the importance of convective overshoot would produce an overestimate of convective overshoot.