A type of Population II Cepheid variable; abbr. CW. They are giants that undergo radial pulsation in the fundamental mode, with periods from 0.8 to 35 days and amplitudes of 0.3–1.2 mag. Their light-curves generally resemble those of the classical Cepheids, particularly if they have a period of 3–10 days, but outside this range the curves have different amplitudes or shapes. In addition, W Virginis stars have lower masses (0.4–0.6 solar masses) than classical Cepheids and obey a different period–luminosity relation, with absolute magnitudes that are fainter by 0.7–2 mag. There are two subtypes: CWA, with periods of more than 8 days, which includes W Virginis itself; and CWB, with periods of less than 8 days. The latter are sometimes known as BL Herculis stars. W Virginis varies between magnitudes 9.5 and 10.8 every 17.27 days.