W Virginis star

A W Virginis star, also known as a Type II Cepheid variable, is a Population II pulsating variable with a period of 1 to 35 days. Although W Vir stars show a period-luminosity relation, which is characteristic of Cepheids, the relation is different than that of Type I Cepheids, or Delta Cephei stars. W Vir stars are typically 1.5 magnitudes fainter than their Type I cousins and have a mass of less than 1 solar mass, so that they are clearly at a different evolutionary stage. They also have a distinctive light curve with a variation of 0.3 to 1.2 magnitude and a bump on the decline side. Two subgroups are recognized: long-period W Vir stars, exemplified by W Virginis itself, with periods of greater than 8 days, and short-period W Vir stars, exemplified by BL Herculis, with periods of less than 8 days.