The minimum mass ratio of W ursae majoris binaries

Frederic A. Rasio*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

85 Scopus citations


The minimum mass ratio for tidal stability of a contact binary containing two unevolved main-sequence stars is calculated to be qmin ≃ 0.09 in the case of a mostly radiative primary, and it is higher if an appreciable fraction of the mass lies in a convective envelope. At least one observed system, AW UMa, has a mass ratio just below this value (q = 0.075), implying that, if the system is stable, the primary must be slightly evolved and must have a very shallow convective envelope. Contact binaries with mass ratios significantly below that of AW UMa should not be observed, since they are tidally unstable and quickly merge into a single, rapidly rotating object, on a timescale ∼ 103-104 yr.

Original languageEnglish (US)
Pages (from-to)L41-L43
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - May 1 1995


  • Binaries: close
  • Blue stragglers
  • Hydrodynamics
  • Instabilities
  • Stars: evolution
  • Stars: rotation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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