A W Ursae Majoris variable is a type of eclipsing binary variable star. These stars are close binaries, whose surfaces are in contact with one another. They are termed common envelope binaries because they essentially share material in their outer layers. Through the neck between the two stars mass and heat is transferred between the components. The latter one equalizes the temperatures of the components.
W Ursae Majoris variables are the most common variable stars in the present day Universe. About 1 percent of all stars belonging to this group.
The class is divided into two subclasses: A-type and W-type (L. Binnendijk, Veroeffentlichungen der Remeis-Sternwarte zu Bamberg, Nr. 40., p. 36, 1965) 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 day. The W-types have cooler spectral types of G or K and shorter periods of 0.22 to 0.4 day. The difference between the surface temperatures of the components is less than several hundred [kelvin]s. A new subclass was introduced in 1978: B-type. The B-types have larger surface temperature difference. In 2004 the H (high mass ratio) systems were discovered by Sz. Csizmadia and P. Klagyivik ([Astronomy and Astrophysics], Vol. 426, pp. 1001-1005 (2004)). The H-types have a higher mass ratio than q = 0.72 (q = (secondary's mass)/(primary's mass)) and they have extra angular momentum.
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.
W Ursae Majoris is the prototype of this class.
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