On the Instability Strip of the Cepheid Stars

In this paper we discuss the location of the instability strip in the C-M diagram of Galactic Cepheids with the aid of the 1990 calculations of model Cepheids by Chiosi & Wood. We find that the difficulty pointed out by Fernie (the slope of the empirical red boundary of the strip is different from that customarily assumed) has no reason to occur because it is entirely due to the use of strips having parallel blue and red edges, while in reality they have a different slope. We also examine the number-period distribution expected from different models for intermediate-mass stars. We find it in good agreement with that predicted by stellar models accounting for convective overshoot. This is not possible for models with semiconvection.