AW Ursae Majoris variable, also known as alow mass contact binary, is a type ofeclipsing binaryvariable star. 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 termedcontact binaries because the two stars touch and transfer mass and energy through the connecting neck, although astronomer Robert E. Wilson argues that the term "overcontact" is more appropriate.^[1]

The class is divided into two subclasses: A-type and W-type.^[3] A-type W UMa binaries are composed of two stars both hotter than the Sun, havingspectral types A or F, andperiods 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 hundredkelvins. 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 Szilárd Csizmadia and Peter Klagyivik.^[4] The H-types have a higher mass ratio than${\displaystyle q=0.72}$ (${\displaystyle q}$ = (secondary's mass)/(primary's mass)) and they have extraangular momentum.

These stars were first shown to follow a period-color relation (shorter period systems are redder) byOlin J. Eggen.^[5] In 2012,Terrell, Gross andCooney published a color-survey of 606 W UMa systems in theJohnson-Cousinsphotometric system.^[6]

Their light curves differ from those of classicaleclipsing binaries, undergoing a constant ellipsoidal variation rather than discreteeclipses. This is because the stars are gravitationally distorted by one another, and thus the projected area of the stars is constantly changing. The depths of the brightnessminima are usually equal because both stars have nearly equal surfacetemperatures.

W Ursae Majoris is the prototype of this class.

• OGLE Atlas of Variable Star Light Curves - W Ursae Majoris type binaries

• W Ursae Majoris variables

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