W Ursae Majoris: The Parameters of A Contact Binary

Spectroscopic radial velocity observations of W Ursae Majoris, the prototype contact binary, are reported. The parameters (masses, radii, luminosities) of the system have been determined and are given in Table V. There is a real discrepancy between the spectroscopically determined mass-ratio of W Ursae Majoris and the photometrically determined one. BD + 55° 1351 and W Ursae Majoris share the same space motion which provides an estimate of the distance to the system. A theoretical main-sequence contact binary model is constructed which reproduces the observations.