3 The spectrum of HDE 228766

As already pointed out by previous investigators, the spectrum of HDE 228766 displays a fair number of emission lines (see Fig. 1) that are essentially associated with the secondary (see Sect. 4). The most prominent ones are He II  $\lambda \lambda$ 4686, 10124; He I $\lambda$ 10830; H${\alpha }$; N III  $\lambda \lambda$ 4379, 4634, 4641; N IV  $\lambda \lambda$ 4058, 7103-11, 7123-29 and Si IV  $\lambda \lambda$ 4089, 4116. The spectral region from 4500 to 4540 Å displays numerous faint N III emission lines, but we find no trace of the two "unidentified Of'' emission lines at 4485 and 4504 Å that were recently associated with S IV transitions (Werner & Rauch 2001). The H${\beta }$ and He I $\lambda$ 5876 lines consist of a mixture of absorption and probably P-Cygni type emission lines. The presence of the N V  $\lambda \lambda$ 4603, 4620 lines is also a remarkable feature suggesting a very early spectral type (O3-O4) for the secondary (e.g. Walborn & Fitzpatrick 1990; Walborn 2001). Beside the "typical'' O-star absorption lines (e.g. H$\gamma$; He I $\lambda$ 4471; He II  $\lambda \lambda$ 4200, 4542; ...) there are a number of interstellar absorptions as well as diffuse interstellar bands (DIBs). The strongest interstellar lines in our data are Na  I  $\lambda \lambda$ 5890, 5896; CH⁺ $\lambda$ 4233 and CH $\lambda$ 4300.

The near-IR spectrum of HDE 228766 reveals a rather prominent and probably variable He II $\lambda$ 10124 emission feature that was first reported by Vreux & Andrillat (1979). Whereas this line is commonly observed in emission in the spectra of Wolf-Rayet (WR) stars, He II $\lambda$ 10124 is usually seen in absorption in the spectra of Of stars. Emission in this line has been reported only for some of the hottest and most luminous Of stars. These include Of⁺ stars (Vreux & Andrillat 1979), several OIafpe transition objects (Crowther & Bohannan 1996) as well as peculiar objects such as the Oef stars $\zeta$ Pup and $\lambda$ Cep and the high mass X-ray binaries Cyg X-1 (Conti & Howarth 1999) and HD 153919 (Bohannan & Crowther 1999). The strong He I $\lambda$ 10830 emission was first noted by Andrillat & Vreux (1979) and is also indicative of a large mass loss rate or an extreme wind extension (Conti & Howarth 1999).

The equivalent width (EW) ratios of the usual O-star classification lines He I $\lambda$ 4471 and He II $\lambda$ 4542 (W' = EW (He I  $\lambda~4471)/EW ($He II  $\lambda~4542)$, Conti 1973) point towards an O7 spectral type for both components ( $\log{W'} = -0.03 \pm 0.07$ and $\log{W'} = -0.04 \pm 0.16$ for the primary and secondary respectively). The presence of Si IV  $\lambda \lambda$ 4089, 4116 in emission in addition to the "classical'' N III  $\lambda \lambda$ 4634-41 and He II $\lambda$ 4686 Of emission lines indicates an Of⁺ classification (see e.g. Walborn & Fitzpatrick 1990) for the secondary star. Though these results are in crude agreement with the classification proposed by MC77 (O7.5 + O5.5f) based on photographic spectra, we caution that the spectral types are rather uncertain due to the serious blending of the classification lines of both stars. Moreover, an O7 classification for the secondary is obviously at odds with the early spectral type implied by the presence of the N IV and N V lines (see also Sect. 6.1).

At this stage, it is worth pointing out that the He I $\lambda$ 4471 absorption is likely to be affected by wind emission. In fact, the He I $\lambda$ 5876 line appears as a mixture of emission and absorption features and a slightly weaker emission at He I $\lambda$ 4471 could essentially remain undetected though it would affect the EWs of this line in the spectrum of both stars. The He II $\lambda$ 4542 line in the spectrum of the secondary might not be of purely photospheric origin either. Indeed, our orbital solution (see the forthcoming Sect. 4) reveals a surprisingly large difference of 150 km s^-1 between the apparent systemic velocities of the two stars with the secondary being the star with the more negative $\gamma$. Since most of the emission lines are also associated with the secondary, the blueshift of the secondary's He II $\lambda$ 4542 absorption could be the result of the strong wind of this star. In this context, Crowther & Bohannan (1997) showed that in extreme Of stars, the usual diagnostic lines such as He II  $\lambda \lambda$ 4542, 5412 are significantly affected by wind contamination and as a consequence the distinction between the most extreme Of stars and the least extreme WN stars becomes a rather tricky issue. It is worth noting that Hackwell et al. (1974) did not find a significant infrared excess in the spectrum of HDE 228766. Such an excess is often seen for Wolf-Rayet stars and is attributed to free-free emission from the dense stellar wind. The relative weakness of the infrared free-free emission in HDE 228766 is consistent with an Of or a weak-lined WN spectral type for the secondary.

Figure 2: Examples of the blue-violet spectra of HDE 228766 as a function of phase illustrating the severe blending of the absorption lines even around $\phi = 0.2$. Note also the numerous weaker N III emission lines.