P73	July 2002

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
² Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom
³ Departemento de Astronomia, Universidad de Guanajuato, Apartado 144, 36000 Guanajuato Gto, Mexico

^* Research Associate FNRS, Belgium

⁺ Based on observations collected at the Observatoire de Haute Provence (France) and the Observatorio Astronomico Nacional of San Pedro Martir (Mexico).

Abstract

     We use an extensive set of spectroscopic observations to reinvestigate the properties of the massive binary HDE228766. Conventional classification criteria suggest that HDE228766 consists of an O7 primary and an Of+ secondary. However, several spectral features of the secondary, such as the simultaneous presence of N III, N IV and N V emissions, make it a rather unusual object. We find that the orbital motion of the secondary is probably best described by the radial velocities of the narrow N III emission lines. Our orbital solution yields m₁ sin³ i = 31.7 and m₂ sin³ i = 25.5 M for the primary and secondary respectively. The He II 4542 absorption in the secondary's spectrum appears considerably blue-shifted with respect to the narrow emission lines, indicating that the absorption is probably formed in the accelerating part of the secondary's wind. We use a tomographic technique to investigate the profile variability of the broader emission lines. In addition to a strong emission from the secondary, the H line displays a weak emission feature that is probably associated with a wind interaction region located near the surface of the primary star. Finally, our analysis of the spectrum with a non-LTE code indicates that the secondary is an evolved object that exhibits some CNO processed material in its atmosphere and has a large mass loss rate. Assuming a distance of 3.5 kpc (which follows from adopting M_{V, s} = -6 for the secondary) we infer a mass loss rate ~10^-5 M yr^-1 for the secondary. Our results suggest that HDE228766 could be in a post-Roche lobe overflow stage. The secondary may be classified as WN8ha and is currently in a transition stage between a 'normal' Of star and a WNL-type Wolf-Rayet star.