W Virginis is another name for Type II cepheid variable stars. They generally have low metallicities, low masses (less than one solar mass), are on average 1.5 magnitudes fainter than Type I Cepheids, and have pulsation periods of between 1 and 100 days.
Like Type I Cepheid variables, W Virginis stars are located in the instability strip of the Hertzsprung-Russell diagram, and their pulsations result from instabilities that cause their size to change. They also exhibit a period-luminosity relation which is distinct, but works in a similar way to the relation for Type I Cepheids. This means that W Virginis stars can also be used to measure galactic and extragalactic distances.