The orbital elements of HD 191226 have been derived from the 36 radial-velocity measurements listed in Table 1. Among these, 31 were obtained with the CORAVEL spectrovelocimeter at OHP (with an average uncertainty of km s). One has been obtained at the Coudé focus of the 1.52-m telescope at OHP, on a baked IIaO plate (# GA 8139), with a 2.0 nm/mm dispersion. Four radial-velocity measurements obtained by Brown et al. (1990) on the 2.1-m telescope at McDonald Observatory equipped with a Reticon detector have also been used in the orbital solution. All these radial velocities, covering 3.37 orbital cycles, are on the system of IAU standards. The orbital elements listed in Table 2 have been computed with the BS1 program (Nadal et al. 1979) by assigning a weight 1 to the CORAVEL data and 0.25 to the five other measurements. The radial-velocity curve, folded with the orbital period, is presented in Fig. 1. Older data, obtained through the years 1912-1934, were not used in the orbital solution, because their accuracy is not good enough, even to improve the orbital period. The Mount Wilson data have (when available) uncertainties of the order of 2.5 to 3.3 km s, and their variation range (-13 to 35 km s;Abt 1973) appears incompatible with the small value of the semi-amplitude (K = 4.8 km s;Table 2) derived from modern, more accurate measurements. As far as the the radial velocities obtained at Victoria by Harper (1934) are concerned, they are of doubtful quality (both plates are qualified as "weak'') and were therefore not used either.
|Figure 1: The radial-velocity curve of HD 191226, folded with the orbital period. Filled dots refer to OHP data (CORAVEL and Coudé), and open dots to McDonald data|
An estimate of the mass of the companion can be obtained from the mass function listed in Table 2. Adopting a mass of for the S star, which seems reasonable given its M2III spectral type (e.g., Schmidt-Kaler 1982; see also Jorissen et al. 1998), a minimum mass of for the secondary follows from the condition . Taking M1 = 1.5 for the red giant and M2 = 0.6 for its companion yields ,q = M2/M1 = 0.4 and A1 = 121 Gm, or in terms of orbital separation, A = A1 + A2 = A1 (1 + 1/q) = 2.8 AU.
The companion mass is thus compatible with that of a white dwarf (WD). Observations in the UV with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope indeed reveal the presence of a WD with K (Ake 1997). These results will be discussed further in Sect. 4.
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