Ultraviolet photometry from the Orbiting Astronomical Observatory. XIX. Atmospheric properties of the detached binaries VV Orionis and MR Cygni.

A series of OAO 2 light curves for VV Ori covering the wavelength range 13004300 A is presented. Analysis of these light curves using light-curve synthesis techniques leads to a mass ratio for the system consistent with radial-velocity observations and to limb darkening consistent with theory. The gravity darkening for the primary, however, is much less (g = 0.3) than theoretically expected for stars with radiative envelopes. Analysis of light curves for MR Cyg, a system similar to VV Ori, indicates that the gravity darkening of MR Cyg could be as low as that found for VY Ori. The mass-luminosity relation applied to this system, however, implies that the primary is probably fully gravity darkened or that the stars may not be adequately represented by Roche potentials. For both of these systems the wavelength dependence of the reflection effect is qualitatively that which is expected from the predictions of Rucinski, although for VV Ori it is higher than expected in the ultraviolet. Subject headings: eclipsing binaries - early-type stars - stars, individual - ultraviolet