3 Observations and overview of the survey

  Since 1983, more than 2500 measurements have been gathered for the stars of the samples described above. The radial velocities are obtained by usual cross-correlation (cc) and gaussian-fitting techniques (see Baranne et al. 1979 or Duquennoy et al. 1991 for details). An illustrative example of a profile obtained for HD 24035 (cc-dip with the fitted gaussian) is displayed in Fig. 1a.

  

Figure 1: Statistical overview of the combined samples of barium and S stars: a) typical cross-correlation profile (here an observation of HD 24035); b) distribution of individual measurement errors $\varepsilon_i$ (in  kms-1); c) distribution of the number of CORAVEL measurements per star N; d) distribution of spans in days ($\Delta T$) between the first and the last measurements, for every star

The still unpublished individual measurements will be available at the Centre de Données Stellaires (CDS) in Strasbourg or on our dedicated web page (obswww.unige.ch/$\sim$udry/cine/barium/barium.html) and thus will not appear here.

The mean precision of the measurements is about 0.3 kms^-1 (more than 76% of the measurements between 0.25 and 0.4 kms^-1) as shown in Fig. 1b displaying the distribution of individual errors $\varepsilon_i$. S stars with non-orbital radial-velocity variations usually present non-gaussian cc-dips (see Sect. 5.1) leading to non-realistic error estimates that slightly swell the upper tail of the distribution. Stars with broad cc-dip widths act in the same way.

The distributions of the number of observations and of the time span of the observations for every star are given in Figs. 1c and 1d, respectively. They illustrate the large observational effort devoted to these programmes. The median value for the number of observations per star is $\overline{N}=19$ which permits a good estimate of the orbital parameters. The typical span ($\overline{\Delta T} =3398$d) is about 10 years ensuring a good completeness of the orbital periods up to this value.

Further information on the CORAVEL observation and reduction techniques can be found in Duquennoy et al. (1991).