W Ursae Majoris: Mass-Ratio Discrepancy, Third Body and Age

The discrepancy between the spectroscopically and photometrically determined mass-ratios of W Ursae Majoris, and the observed quasi-sinusoidal period variations may be understood if there is a third body in the system which contributes about 4 per cent of the light at maximum. The third body has a mass of about 0.2 $M_\odot$ and could be a Hayashi star. The time scale for its contraction to the main-sequence (5 × 108 yr) is of the same order as an estimate of the age of W Ursae Majoris (1–5 × 108 yr) deduced from the time since coincidence of W Ursae Majoris and its proper motion companion BD + 55° 1351. The inferred age of W Ursae Majoris may be significant for contact binary evolution theory. Several observational tests of the third-body hypothesis are proposed. The possibility that the period changes are caused by mass exchange, mass loss or apsidal motion cannot be ruled out and these mechanisms are compared with the third-body hypothesis.