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A W Ursae Majoris variable is a type of eclipsing binary (opens in a new tab) variable star (opens in a new tab). These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one another. They are termed contact binaries (opens in a new tab) because the two stars touch and transfer mass and energy through the connecting neck, although astronomer R.E. Wilson argues that the term “overcontact” is more appropriate.

W Ursae Majoris variables are the most common variable stars in the present day Universe. About 1 percent of all stars belong 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 (opens in a new tab) A or F, and periods (opens in a new tab) 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 kelvins (opens in a new tab). 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 (opens in a new tab), Vol. 426, pp. 1001-1005 (2004)).

Their light curves differ from those of classical eclipsing binaries (opens in a new tab), undergoing a constant ellipsoidal variation rather than discrete eclipses (opens in a new tab). 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 (opens in a new tab) are usually equal because both stars have nearly equal luminosities (opens in a new tab). W Ursae Majoris (opens in a new tab) is the prototype of this class.

* EW = W UMa. Components almost touching, and primary & secondary eclipses near-equal. Periods < 1 day.