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Astron. Astrophys. 362, 189-198 (2000)

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3. Determination of the orbital elements

Since the velocity amplitude of the orbital motion is comparable to that of the pulsation motion, it is necessary to remove the orbital component from the total variation before the pulsation motion analysis. We first considered the orbital ephemerides provided in the literature by successively Kodaira (1971), Abt & Levy (1978) and Mulliss (1996). However, mainly due to a bad phase coverage and because pulsations were involved, a clear dispersion appears between these ephemerides, in particular concerning the eccentricity ([FORMULA] for Abt & Levy (1978) and [FORMULA] for Mulliss (1996)). This parameter is very important since it determines the shape of the orbital velocity curve to remove from our data. We thus decided to use all previous spectroscopic data available in the literature to compute a new ephemeris. In order to reduce the effect of the pulsation component, we considered only the average velocity of a given night if more than one observation was obtained during that night. The more scattered data are those of Kodaira (1971). This may be due to the low dispersion used (20 Å.mm-1), and to the measured lines (Balmer lines, certainly blended, subject to Stark effect and having wide profiles). Nevertheless, because his data were well distributed during his observing run, we took them into account with the exception of 4 points, which were too far away from any other observation. The parameters of the binary system were thus determined using 133 data points, representing the average of 1002 measurements. They are given in Table 1. Assuming a mass of 7 [FORMULA] for the primary, the mass function provided here implies an upper limit of 0.4 [FORMULA] for the companion. This result agrees with the white dwarf usually suspected (Peters & Aller 1970). The binary orbit together with the data (each night being averaged) are represented in Fig. 6.

[FIGURE] Fig. 6. Plot of the orbital radial velocity curve together with different data sets: Petrie & Petrie (1939): black dots; Kodaira (1971): circles; Abt & Levy (1978): crosses; Chapellier et al. (1987): [FORMULA]; Le Contel et al. (1987): open squares; 1987 data (this paper): open crosses; Paper I: black squares; Mulliss (1996): black stars; 1995 data (this paper): black triangles


Table 1. Parameters of the binary orbit. P is the orbital period, [FORMULA] is the center-of-mass velocity, K is the orbital velocity amplitude, T is the epoch of periastron passage, e is the eccentricity, [FORMULA] is the longitude of periastron, [FORMULA] is the projection of the semi-major axis and [FORMULA] is the mass function

Our orbital elements differ significantly from those given by Abt & Levy (1978); for instance, our period and eccentricity are out from their proposed range. Conversely, our elements are more or less within the error bars given by Mulliss (1996), except the eccentricity and the orbital amplitude which are respectively significantly larger and lower. Note that our time basis is much more important than Mulliss's one (1996): about 58 years compared to a bit more than 1 year. It is also larger than that of Abt & Levy (1978), who used only about 100 data spread over 38 years. This explains why our confidence level is rather high. These new elements were used to subtract the orbital motion from the different velocity sets considered in the following sections.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100