The evolved early-type binary HDE 228766 revisited

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 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 m(1) sin(3) i = 31:7 and m(2) sin(3) i = 25:5 M. for the primary and secondary respectively. The He II lambda 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 Halpha 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 (M) over dot similar or equal to 10(5) M. yr(-1) 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 staranda WNL-type Wolf-Rayet star.