Radius Variation and Population Type of Cepheid Variables

The method of Wesselink for the determination of the radii of pulsating variables is applied to the southern cepheids κ Pavonis, β Doradus and l Carinae. The radius of β Doradus is found to be abnormally large for its period. The results are combined with the radii of other cepheids found by previous workers using the same method and a plot of relative radius amplitude against period is made. It is apparent that, for classical cepheids, the light variations are mainly due to changes in effective temperature, and the relative radius amplitude increases slowly with period. However, for the type II cepheids, the relative radius amplitude increases rapidly with period until for stars of the longest period W Virginis type the variation in radius is of the order of the mean radius of the star itself.