1 Introduction

Several samples of chemically-peculiar red giants (PRG) were monitored in radial velocity with the aim of deriving their binary frequency. The spectroscopic-binary orbits presented here and in a companion paper (Udry et al. 1998, Paper II) belong to a systematic survey of barium and Tc-poor S stars undertaken to gain insight into the formation process of these stars.

Barium stars and their Pop. II counterparts, the CH stars, are not evolved enough to have produced on their own the observed overabundances of elements heavier than Fe (e.g. Lambert 1985), usually associated with the nucleosynthesis occurring during He-burning thermal pulses on the asymptotic giant branch (AGB). The binary nature of these stars accounts for the observed chemical peculiarities through mass transfer across the binary system (McClure et al. 1980; McClure 1983). The exact mass-transfer mode is still a matter of debate but is turning towards a solution involving Roche-lobe overflow or wind accretion, depending on the orbital period (Han et al. 1995).

Contrarily to Tc-rich S stars which are able to synthesize heavy elements on their own, Tc-poor S stars are believed to be the cool descendants of barium stars and thus also owe their chemical peculiarities to binarity (Jorissen & Mayor 1992; Ake 1997; Van Eck et al. 1997).

The 46 new (43) or updated (3) spectroscopic orbits of barium stars (+ 6 Ba stars with $P_{\rm min}$ determinations) and the 10 new orbits of S stars (+ 6 binary S stars with $P_{\rm min}$ determinations) reported in this paper and in Paper II, combined with a few other newly available orbits (see Refs. in Jorissen et al. 1998), provide an unequalled sample to constrain the binary evolution channels relevant for barium stars, as identified by Han et al. (1995) using only 17 barium-star orbits. It also allows us to rediscuss the evolutionary link between barium and Tc-poor S stars. This is done in a companion paper (Jorissen et al. 1998).

In order not to bias the S-star sample towards low luminosities, a small sample of S Miras, SC and Tc-poor C stars were also monitored. However, the radial-velocity jitter associated with atmospheric motions (envelope pulsation, large convective cells, shocks) induces non-orbital radial-velocity variations that limit our ability to detect binaries and, a fortiori, to derive their orbital elements, at a given level of measurement precision. Illustrative examples will be discussed at the end of the paper.

This paper is organized as follows. Section 2 describes the stellar samples. The observations are briefly discussed and a short statistical overview of the survey is given in Sect. 3. Section 4 provides the orbital elements and the radial-velocity curves. Some stars are also commented individually. Mira S, SC and C stars with radial-velocity variations associated or not with an orbital motion are discussed in Sect. 5.