W UMa (W Ursae Majoris) is the prototype of a class of contact binary variable stars.
The W Ursae Majoris stars are close binary stars, whose surfaces are in contact with one another. They are termed "common envelope binaries" because they essentially share material in their outer layers.
The class is divided into two subclasses: A-type and W-type. A-type W UMa binaries are composed of two stars both hotter than the Sun, having spectral types A or F, and periods of 0.4 to 0.8 days. The W-types have cooler spectral types of G or K and shorter periods of 0.22 to 0.4 days.
Their light curves differ from those of classical eclipsing binaries, undergoing a constant ellipsoidal variation rather than discrete eclipses. This is because the stars are in physical contact and thus constantly eclipse one another, and also because the stars are gravitationally distorted by one another. The depths of the brightness minima are usually equal because both stars have nearly equal luminosities.