Astron. Astrophys. 343, 273-280 (1999)

1. Introduction

Abt (1970) reports that the number of binary systems in clusters increases when the average stellar rotational velocity decreases. Since chemically peculiar (CP) stars usually present smaller rotational velocities than normal main sequence stars, the incidence of binaries among CP stars is expected to be higher than among normal main sequence stars. This aspect of CP stars could be related to the atmospheric stability that is necessary to explain the observed non-homogeneous distribution of chemical elements on the stellar surface.

In contradiction to the correlation between rotational velocity and binary frequency found by Abt (1970), the incidence of binary systems among CP stars appears to be lower than among normal stars (Jaschek & Jaschek 1976). Jaschek & Gomez (1970) found an incidence of 475% of binaries which is rather constant from B to M main sequence stars. Gerbaldi et al. (1985) determined the incidence of binaries among CP star subgroups and concluded that: a ) there is a deficiency of binaries among He-weak, Si, SiCr and SiSr stars, b ) the incidence of binaries among the coolest CP stars and the HgMn stars is not different from that among normal main sequence stars. For cool CP stars a different result was obtained by North (1993), who performed a Coravel survey and estimated an incidence of spectroscopic binaries equal to 19% and not larger than 30%.

A further difference between CP and normal main sequence stars is the distribution of eccentricity as a function of the orbital period (Jaschek & Jaschek 1976). Gerbaldi et al. (1985) found that there is a lack of circular orbits for peculiar stars. The previous period-eccentricity relation is based on a relatively small number of cases: 14 cool CP stars, 22 HgMn stars, 12 Si stars and 1 He-weak star.

With the aim to increase the number of CP stars with known orbital parameters and to extend the period-eccentricity relation to helium peculiar stars, we have carried out spectroscopic observations and determined the orbital elements of 2 cool CP stars, 1 Si star, 2 helium-weak stars and 2 helium-strong stars.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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