W Ursae Majoris variable


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.[1]

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 light curve for W Ursae Majoris, the class prototype, plotted from TESS data[2]

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, 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. [4] The H-types have a higher mass ratio than ( = (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.[5] In 2012, Terrell, Gross and Cooney published a color-survey of 606 W UMa systems in the Johnson-Cousins photometric system.[6]

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)[a] Apparent magnitude (Minimum)[a] Range of magnitude Period Spectral types
(eclipsing components)
AB And Andromeda   10.40 11.27 0.87 0.3319 d G5 G5V
S Ant Antlia H.M.Paul, 1891 6.27 6.83 0.56 0.6483489 d A9V A9V
44 (or i) Boo B Boötes   5.8 6.4 0.6 0.2678159 d G2V G2V Triple system, A being non-variable
TU Boo Boötes   11.8 12.5 0.7 0.324 d
VW Cep Cepheus   7.23 7.68 0.45 0.278 d G5 K0Ve
WZ Cep Cepheus   11.4 12.0 0.6 0.41744 d F5 Possible triple system
ε CrA Corona Australis   4.74 5.0 0.26 0.5914264 d
SX Crv Corvus 8.99 9.25 0.26 0.32 d F7V ?
V1191 Cyg Cygnus 10.82 11.15 0.33 0.31 d F6V G5V
V571 Dra Draco Barquin, 2018 14.43 14.77 0.34 0.428988 d
XY Leo Leo   9.45 9.93 0.48 0.284 d K0V K0
CE Leo Leo   11.8 12.6 0.8 0.303 d
TV Pic Pictor Verschuren, 1987 7.37 7.53 0.16 0.85 d A2V A9-F0V
Y Sex Sextans 9.81 10.23 0.42 0.42 d F3/5V[4] or F5/6V Possibly two substellar companions
W UMa Ursa Major   7.75 8.48 0.73 0.3336 d F8Vp F8Vp Prototype, possible triple system


  1. ^ a b (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))


  1. ^ Wilson, R. E. (2001). "Binary Star Morphology and the Name Overcontact". Information Bulletin on Variable Stars. 5076: 1. Bibcode:2001IBVS.5076....1W.
  2. ^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  3. ^ Remeis-Sternwarte (1965). "Veraenderlichen Colloquium : 3 : 1965". Kleine Veroeffentlichungen der Remeis-Sternwarte Bamberg. Bibcode:1965veco.conf.....R.
  4. ^ Sz. Csizmadia and P. Klagyivik (18 October 2004). "On the properties of contact binary stars". Astronomy and Astrophysics. 426 (3): 1001–1005. arXiv:astro-ph/0408049. Bibcode:2004A&A...426.1001C. doi:10.1051/0004-6361:20040430.
  5. ^ "Contact Binaries II". Memoirs of the Royal Astronomical Society. 70: 111. 1967. Bibcode:1967MmRAS..70..111E.
  6. ^ Terrell, Dirk; Gross, John; Cooney, Walter (2012). "A BVRCIC Survey of W Ursae Majoris Binaries". Astronomical Journal. 143 (4): 99. arXiv:1202.3111. Bibcode:2012AJ....143...99T. doi:10.1088/0004-6256/143/4/99. S2CID 118622542.