Masses for Vela X-1 and other X-ray binaries

A detailed analysis of the optical light curve and other previously known observations of the X-ray binary Vela X-1 (3U 0900--40) is reported. All constraints are consistent only with a projected orbital velocity of the X-ray source of between 200 and 300 km s-1. The allowed ranges of the masses and other parameters is estimated. If the mass of the X-ray source is less than the Chandrasekhar limit for white dwarfs, then the optical star must have an unusually small mass-to-radius ratio for its spectral type. The importance of the X-ray eclipse duration in determining binary parameters is emphasized, and some new results are presented from analyses of the observations of 3U 1700--37 and other X-ray binaries