Period-Luminosity Relations of Population II Cepheids

The pulsation mode of Type II Cepheid variable stars is investigated with the aid of the fundamental period-mean density pulsation equation. In light of the dependence of Q0 and Q1 on the mass-to-radius ratio M/R, it is shown that it is likely that all of the Type II Cepheids are fundamental pulsators. Period- luminosity relations are derived from MV and MB values of variables in globular clusters. Separate P-L relations are derived for the stars with P<10d (BL Her stars) and the stars with P>l0d (W Vir stars). The steep slope of the P-L relation of the W Vir stars suggests a possible dependence of the absolute magnitude of the variables on mass.