P29	September 1998

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
² Kitt Peak National Observatory, National Optical Astronomy Observatories⁺⁺, P.O. Box 26732, Tucson AZ 85726-6732, USA

⁺ All the observations reported here were obtained at the Observatoire de Haute-Provence and at the Kitt Peak National Observatory.
⁺⁺ The National Optical Observatories are operated by the Association of Universities for Research in Astronomy, Inc (AURA) under cooperative agreement with the National Science Fondation.

Abstract

     We present the analysis of an extensive set of spectroscopic observations of the mysterious early-type binary BD+40°4220. A new orbital solution is derived from the radial velocities of the absorption lines. We confirm that the secondary star is highly overluminous for its mass. The absorption lines of both components display phase-locked profile variations with some of the secondary's lines going into emission between = 0.20 and = 0.55. A detailed investigation of the profile variability of the He II  4686 emission line reveals that the pattern of variability of this line is very stable. It is shown that part of the He II  4686 emission is produced in the wind interaction region between the stars. Most of the emission lines in the visual spectrum of BD+40°4220 display variations that are reminiscent of those observed on the He II  4686 line, pointing towards a similar origin.

     We present numerical simulations of emission line profiles in a colliding winds binary, showing that an important part of the variability observed in BD+40°4220 can be explained by a colliding winds phenomenon. The properties of the wind interaction region can be accounted for if we assume that the secondary star is an evolved object, most probably some kind of Ofpe/WN9 transition star with a mass loss rate of ~ 5.5 10^-6 Myr^-1. We finally discuss the fundamental parameters of the binary, concluding that mass transfer must have played a crucial role in the evolution of this system.