1 Introduction

HDE 228766 (=BD $+36^{\circ}$ 3991) was first reported to be a spectroscopic binary with a period of about 10.6 d by Hiltner (1951). In addition to absorption lines of H  I, He I and He  II, the spectrograms collected by Hiltner revealed the presence of emission lines of He II $\lambda$ 4686 and N IV $\lambda$ 4058 and this author classified HDE 228766 thus as "WN7 or slightly earlier''. The Wolf-Rayet classification of this system was soon questioned by several authors. In fact, Roman (1951) noted that the emission lines in the spectrum of HDE 228766 were rather narrow, unlike what was observed for typical Wolf-Rayet stars, and therefore Roman suggested that HDE 228766 should be called an Of star. Hiltner & Schild (1966) reclassified HDE 228766 as an Of-type system and this result was also supported by Walborn (1973) who suggested that the system consists of an early Of star and a rapidely rotating late O supergiant. Very recently, Walborn et al. (2002) classified the combined spectrum of HDE 228766 as O3.5 If^*. A leap forward in the study of this system came with the work of Massey & Conti (1977, hereafter MC77). These authors derived an O7.5 + O5.5 f classification for the primary and secondary respectively. More importantly, they succeeded for the first time to resolve some of the absorption lines in the spectrum of HDE 228766 into two components. In fact, earlier investigators (Hiltner 1951; Hart 1957) failed to resolve these lines and assigned the entire absorption spectrum to the late O primary star. The older orbital solutions hence only provided upper limits on the true orbital motion of the late O component. MC77 found the apparent systemic velocity of the secondary's absorptions to be considerably blue-shifted relative to the primary, while the He II $\lambda$ 4686 emission line appeared red-shifted.

Beside its interest for our understanding of stellar evolution, HDE 228766 could also be considered a promising target for studies of stellar wind interactions. The fact that this binary was quoted as a moderately bright X-ray source (Chlebowski et al. 1989) makes it indeed a good candidate for a colliding wind phenomenon. These considerations prompted us to re-investigate the properties of this binary system with a particular emphasis on the search for phase-locked emission line profile variability that is often associated with a wind-wind collision.

In this paper, we present an extensive set of new spectroscopic observations of HDE 228766. In Sect. 3, we discuss the spectral features of HDE 228766. Radial velocity measurements are used in Sect. 4 to derive new orbital solutions. The emission line profile variability is analysed in Sect. 5. Finally, a discussion of the fundamental parameters of this system and of the wind interaction phenomenon is presented in Sect. 6.