The structure of the Cepheid instability strip

About 100 classical Cepheids having color excesses on a homogeneous system with standard errors <= 0.02 mag are used with the Feast-Walker period-luminosity-color relation to study the distribution of such stars in the instability strip. It is found that [B-V](mag) is a better indicator of mean effective temperature than is [B](i) - [V-i](i). The blue edge of the color-magnitude distribution is consistent with the theoretical blue edge for Y = 0.28 and Z = 0.02. Although the highest amplitude stars are found near the center of the period-color array, high- and low-amplitude stars can intermingle, and both kinds are to be found near the edges of the distribution. The same is true on the C-M array. Finally, it is pointed out that the Cepheids do not populate the instability strip uniformly if the red edge is taken to be parallel to the theoretical blue edge. Rather, the local instability region runs as a parallelogram in the C-M array from the theoretical blue edge upward and to the red.