DN ORIONIS - A LONG-PERIOD MASS-TRANSFERRING SLIGHTLY DETACHED ALGOL BINARY

We present new full-orbit spectroscopic and I(Kron)yb upsilon u photom etric observations of the totally eclipsing long-period Algol-type bin ary DN Orionis. We carry out new radial velocity and light solutions, which include the rapid rotation of the hotter star. We show that the cool component does not quite fill its Roche lobe, though an accretion disk is present around the hotter star (as shown by double-peaked Ho emission and excess O I lambda 7774 absorption). Simple Roche-lobe ove rflow therefore cannot explain this circumstellar matter. Absolute sys tem parameters are obtained from the cool-star radial velocity amplitu de and a fairly well determined photometric mass ratio. During primary eclipse totality, we find about 0.3 mag of excess ultraviolet emissio n radiated by the uneclipsed outer edges of this disk. DN Ori joins se veral other long-period Algols with a slightly detached configuration, a supersynchronously rotating hot star, and a low-mass evolved cool s tar. (C) 1995 American Astronomical Society.