W Virginis variable
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A W Virginis variable is a variable star similar to a Cepheid variable. However, using the same formula to calculate its distance will yield a value that is too small, a mistake that Edwin Hubble made while observing M31.
W Virginis stars differ from Cepheids because they are Population II stars, and so have a lower mass and lower metallicity than our Sun. Their luminosity is, on average, about 1.5 magnitudes less than classical Cepheids. The period varies between 1 and 60 days.
W Virginis are also called Type II Cepheids.