The Cepheid instability strip

Calculations are performed to theoretically determine the location of the red edge of the Cepheid instability strip. As for the RR Lyrae stars, the color width of the instability strip is primarily an indication of helium abundance, with little dependence on mass or metal abundance. There is some dependence on luminosity over large enough ranges. A comparison of the theoretical and observed instability strips is given. The results of this comparison indicate that our RR Lyrae red edges were probably not systematically in error, and that the helium abundance estimated for the Cepheid surface layers is normal.