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 of short periods, rather eccentric orbits, and companions of low mass. Unfortunately these results are based on a relatively small ( 50) number of CP-star binary systems with known orbital parameters and a similar analysis 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 component is a CP star, we have performed spectroscopic observations and determined the orbital elements for seven of these systems. Of these we have included two helium-weak and two helium-strong stars. The values found for the orbital elements confirm the de- ficiency of short periods and the lack of circular orbits for CP stars: only HD 15144 has an orbital 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 deter- mined orbital periods longer than 12 days and large eccentricity values (0.26-0.40). As O A star binary systems have circular orbits only when their orbital periods are less than two days, we conclude that CP- star binary systems are characterised only by long orbital peri- ods with respect to normal stars. Probably a small component separation and/or a massive companion, which are associated with short orbital periods, is responsible for such a strong atmo- spheric 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 36485 is only 8 km s 1 , one of the smallest known values for a CP star, which appears to be consistent with the small ( 10) inclination of the rotational axis.