2002A&A...392..563R

We use an extensive set of spectroscopic observations to reinvestigate the properties of the massive binary HDE 228766. Conventional classification criteria suggest that HDE 228766 consists of an O7 primary and an Of⁺ secondary. However, several spectral features of the secondary, such as the simultaneous presence of NIII, 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 m₁sin ³i=31.7 and m₂sin³i=25.5M_☉ for the primary and secondary respectively. The He II λ4542 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 Hα 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 3.5kpc (which follows from adopting M_V, s_=-6 for the secondary) we infer {dot}(M)≃10^–5M_☉/yr for the secondary. Our results suggest that HDE 228766 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.