1 Introduction

Multiplicity plays an essential, though as yet poorly understood, rôle in the formation and maybe the evolution of A-type chemically peculiar stars. While the rate of binaries tends to be rather small among the magnetic Ap stars, with an especially strong deficit of SB2 binaries, it is very high among the Am stars, which on the contrary, are very often found in SB2 systems. There is also a conspicuous lack of orbital periods smaller than 3 days among the magnetic Ap stars, as shown e.g. in the review by Gerbaldi et al. (1985). The multiplicity of Am stars has been explored by Abt & Levy (1985), and their work showed that all Am stars are not necessarily found in binaries (contrary to what earlier results suggested), or at least not in short-period binaries (even though "short'' means P < 1000 days). This indicates that duplicity is not an absolute prerequisite for the Am peculiarity to appear, so that the slow rotation of these stars, which presumably allows radiative diffusion to work in their atmosphere (Michaud et al. 1983), is not always due to tidal friction (Abt 1985; Abt & Levy 1985).

In order to increase the as yet insufficient statistics, a radial velocity survey of cool, magnetic Ap stars has been initiated in 1985 using the CORAVEL scanner. Some Am stars were also monitored in the course of this programme, for ambiguous classification caused them to be considered as Ap stars. Preliminary results of this survey have been published by North (1994). Observations of Am stars with CORAVEL had been initiated in the early eighties by M. Mayor and W. Benz, with the purpose of determining their projected rotational velocities, but the results were not published. Since this project was not intended to determine the rate of binaries, only few stars have more than 2 or 3 measurements, and HD 96391 was one of them.

In 1992, two of us (JMC and NG) began a radial-velocity survey of all northern Am stars whose metallic type is cooler than or equal to F2, with the purpose of improving our knowledge of their multiplicity. The limit imposed on the spectral type deduced from the metallic lines was chosen because of CORAVEL's optimum efficiency for cool stars. The stars HD 43478 and 96391 were included in this survey and were therefore measured independently by the Geneva-Lausanne observers as well as by the Toulouse observers. The latter measured also a few Ap stars, and we present here the results based on the merged data of these common stars.

The observations are briefly described in Sect. 2 while the results are presented and discussed in Sects. 3 and 4 for the Ap and Am stars respectively. Extensive use has been made of Renson's (1991) catalogue in this work.

  

Table 1: Orbital parameters of the binaries. For each component, the second line gives the estimated standard deviations of the parameters