W Ursae Majoris variable

Schematic of a W Ursae Majoris variable with a mass ratio of 3. Both stars (the filled regions) overfill their Roche lobes (enclosed by the black lines).

A W Ursae Majoris variable, also known as a low mass contact binary, is a type of eclipsing binary variable 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 termed contact 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.

A light curve for W Ursae Majoris, the class prototype, plotted from TESS data

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 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. 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. 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.

These stars were first shown to follow a period-color relation (shorter period systems are redder) by Olin J. Eggen. In 2012, Terrell, Gross and Cooney published a color-survey of 606 W UMa systems in the Johnson-Cousins photometric system.

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 gravitationally distorted by one another, and thus the projected area of the stars is constantly changing. The depths of the brightness minima are usually equal because both stars have nearly equal surface temperatures.

W Ursae Majoris is the prototype of this class.

Designation (name)	Constellation	Discovery	Apparent magnitude (Maximum)Template:Efn	Apparent magnitude (Minimum)Template:Efn	Range of magnitude	Period	Spectral types (eclipsing components)		Comment
AB And	Andromeda		Template:Ntsh10.40	Template:Ntsh11.27	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"| G5	style="background:#Template:Color temperature;"| G5V
S Ant	Antlia	H.M.Paul, 1891	Template:Ntsh6.27	Template:Ntsh6.83	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|A9V	style="background:#Template:Color temperature;"|A9V
44 (or i) Boo B	Boötes		Template:Ntsh5.8	Template:Ntsh6.4	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"| G2V	style="background:#Template:Color temperature;"| G2V	Triple system, A being non-variable
TU Boo	Boötes		Template:Ntsh11.8	Template:Ntsh12.5	Template:Nts	Template:Nts d
VW Cep	Cepheus		Template:Ntsh7.23	Template:Ntsh7.68	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|G5	style="background:#Template:Color temperature;"|K0Ve
WZ Cep	Cepheus		Template:Ntsh11.4	Template:Ntsh12.0	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|F5		Possible triple system
ε CrA	Corona Australis		Template:Ntsh4.74	Template:Ntsh5.0	Template:Nts	Template:Nts d
SX Crv	Corvus		Template:Ntsh8.99	Template:Ntsh9.25	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|F7V	?
V1191 Cyg	Cygnus		Template:Ntsh10.82	Template:Ntsh11.15	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|F6V	style="background:#Template:Color temperature;"|G5V
V571 Dra	Draco	Barquin, 2018	Template:Ntsh14.43	Template:Ntsh14.77	Template:Nts	Template:Nts d
XY Leo	Leo		Template:Ntsh9.45	Template:Ntsh9.93	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|K0V	style="background:#Template:Color temperature;"|K0
CE Leo	Leo		Template:Ntsh11.8	Template:Ntsh12.6	Template:Nts	Template:Nts d
TV Pic	Pictor	Verschuren, 1987	Template:Ntsh7.37	Template:Ntsh7.53	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|A2V	style="background:#Template:Color temperature;"|A9-F0V
Y Sex	Sextans		Template:Ntsh9.81	Template:Ntsh10.23	Template:Nts	Template:Nts d	F3/5V[4] or F5/6V		Possibly two substellar companions
W UMa	Ursa Major		Template:Ntsh7.75	Template:Ntsh8.48	Template:Nts	Template:Nts d	style="background:#Template:Color temperature;"|F8Vp	style="background:#Template:Color temperature;"|F8Vp	Prototype, possible triple system

External links

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