Masses and other parameters of the post-common envelope binary be Ursae Majoris

The binary star BE Ursae Majoris is recently emerged from the common envelo pe phase; indeed, the hot sdO/DAO component is the central star of the asso ciated planetary nebula. As such, BE UMa represents an important test case of stellar evolution theory. Using the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS), we measured the radial velocity ampli tude of the He II lambda 1640 absorption line from the sdO/DAO component of this eclipsing system. Combining our results with those of Crampton, Cowle y, & Hutchings, we determine stellar masses in units of solar mass as follo ws: for the sdO, the mass is 0.70 +/- 0.07, and that of the secondary star is 0.36 +/- 0.07, where we report the 1 sigma value for all errors. The sep aration between the component stars is 7.5 R. +/- 0.5 R. and is insensitive to small changes in inclination angle due to the near edge-on viewing angl e of 84 degrees +/- 1 degrees. Using these values, we modeled the eclipse l ight curve. Our results matched observed UBVR light curves of Wood and cowo rkers only if the modeled secondary star radius of 0.72 R. +/- 0.05 R. has nearly double the radius expected from the main-sequence mass-radius relati on. The secondary star has thus not yet relaxed to thermal equilibrium sinc e the common envelope phase ended similar to 10(4) yr ago. Using the lambda 1640 absorption-line profile and the surrounding continuum, we also were a ble to constrain the sdO helium abundance as log n(He) = -1.1 +/- 0.2 and l og n(Fe) < 1. Our results support the sdO/DAO log g similar to 6.5 surface gravity and T-eff similar to 100,000 K values of Liebert et al. and are con sistent with the post-AGB evolutionary track. Our best estimate of the dist ance to the BE UMa system is 2000 pc.