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Astron. Astrophys. 328, 219-228 (1997)

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1. Introduction

The system [FORMULA]  Vel (HR 3207, HD 68273, WC8+O8 III) contains the closest Wolf-Rayet star, WR 11 (van der Hucht et al. 1988). 1 Its radio flux is strong enough to be recorded at mm and cm wavelengths and therefore, the mass loss rate of the WR star is known accurately (Leitherer & Robert 1991). Due to its binary nature [FORMULA]  Vel is one of the few systems for which it is also possible to measure the mass of the WR star. Therefore, [FORMULA]  Vel is an important system for the calibration of the relation between the WR mass and the WR mass loss rate (Langer 1989). This relation is crucial for evolutionary calculations of the Wolf-Rayet phase.

In view of [FORMULA]  Vel's magnitude, [FORMULA], we would expect that its orbit had been measured accurately long ago. However, only 14 years ago, when Pike et al. (1983) published their orbit solution, they wrote: "To resolve an outstanding disagreement on the period of the WC8+O9 I spectroscopic binary [FORMULA]  Velorum , a number of archival plates has been retrieved and measured. A period of 78.52 days has been found, but differences with some other elements have now appeared with serious consequences for the masses of the components and impending analysis of the WR wind."

The analysis of Pike et al. (1983) has apparently been superseded by the work of Moffat et al. (1986). The results of Pike et al. (1983) and of Moffat et al. (1986) are contradictory and the latter solution was accepted as the more reliable because it is based on observations with a good coverage of the whole orbital period and because the resulting mass of the O star agreed better with the classification as a supergiant. The latest investigation of [FORMULA]  Vel 's orbit is that of Stickland & Lloyd (1990), who analyzed IUE observations. Due to the relatively low S/N of these data, their solution had RMS residuals inferior to previous results. Therefore, they concluded: "We cannot claim to have improved the situation with regard to the definitive orbital elements of [FORMULA]  Vel , and indeed, our support for a small velocity amplitude for the O star may be seen as simply muddying some already murky waters."

Recently, the measured parallactic distance to [FORMULA]  Vel by the HIPPARCOS satellite has brought a new argument to the discussion of its orbit (van der Hucht et al. 1997; Schaerer et al. 1997). Due to the revised distance the primary has a lower luminosity than thought previously. Now, the situation is reversed: the mass of the Pike et al. (1983) solution is in better agreement with the revised luminosity of the O star.

In the present paper we address the orbital motion of [FORMULA]  Vel based on a large data set of new spectroscopic observations. The observations are described in the next section. In Sect. 3 we derive the period by combining our data with that of Perrine (1920) and Pike et al. (1983). In Sect. 4 and 5 we measure the orbital elements and in Sect. 6 we investigate a peculiarity revealed by our observations: a phase shift of the C III/IV   [FORMULA] line complex. In Sect. 7 the inclination of the system is determined. In the last section, we compare our results with previous determinations and discuss systematic influences on the measured amplitudes.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998