Orbital elements of binary systems with a chemically peculiar star

When binary systems with a chemically peculiar (CP) star are compared with normal-star binary systems, they present: a lower incidence, a deficiency o f short periods, rather eccentric orbits, and companions of low mass. Unfor tunately these results are based on a relatively small (similar to 50) numb er of CP-star binary systems with known orbital parameters and a similar an alysis has not yet been carried out for helium-peculiar stars, as there is only one helium-weak star with known orbital elements. With the aim to contribute to the study of binary systems whose brightest c omponent is a CP star, we have performed spectroscopic observations and det ermined the orbital elements for seven of these systems. Of these we have i ncluded two helium-weak and two helium-strong stars. The values found for the orbital elements confirm the deficiency of shea pe riods and the lack of circular orbits for CP stars: only HD 15144 has an or bital period shorter than 3 days (the orbital period distribution of normal stars peaks at 3 days) and a circular orbit. As to helium-peculiar stars, we have determined orbital periods longer than 12 days and large eccentrici ty values (0.26-0.40). As O-A star binary systems have circular orbits only when their orbital per iods are less than two days, we conclude that CP-star binary systems are ch aracterised only by long orbital periods with respect to normal stars. prob ably a small component separation and/or a massive companion, which are ass ociated with short orbital periods, is responsible for such a strong atmosp heric mass motion on the stellar surface to prevent the element separation which is at the basis of the CP star phenomenon. The amplitude of the radial velocity curve of the helium-strong star HD 364 85 is only 8kms(-1), one of the smallest known values for a CP star, which appears to be consistent with the small (similar to 10 degrees) inclination of the rotational axis.