ON THE DEFICIENCY OF 8-10 DAY GALACTIC CEPHEIDS

The Galactic Cepheid period histogram has a strong dip between 8 and 1 0 days that has defied an explanation based on evolutionary and linear pulsation studies. We show here that this deficiency is caused by the instability of the nonlinear fundamental pulsation cycle in this peri od range. The strong metallicity dependence of this instability is con sistent with the absence of a corresponding minimum in the Magellanic Cloud data. Our results also suggest that the Galactic Cepheids must h ave a large spread in metallicity.