1 Introduction

To date about 80 planetary candidates have been discovered. Their orbits show a widely-spread distribution of orbital parameters such as eccentricity and orbital period (Udry & Mayor 2001) which might be related to different evolution mechanisms. The minimum mass of the detected planetary companions ranges from $\sim$10 Jupiter masses down to about half the mass of Saturn. Four of the known candidates have sub-Saturnian masses (Jorissen et al. 2001). The lightest planet, HD 83443 c, has been discovered by means of the CORALIE spectrograph and has a mass of only 0.53  $M_{\rm Sat}$ (Mayor et al. 2000).

The technique which underlies all these discoveries is that of precise radial-velocity measurements. It provides us with most orbital parameters. Because this technique does not allow us to constrain the angle of projection $\sin i$ of the orbital plane, only a minimum mass $m_2\sin i$ of the companion can be determined. The radial-velocity variation induced by a planetary companion on its parent star is large if its minimum mass $m_2\sin i$ is large. This fact makes the technique particularly sensitive to high-mass companions. On the other hand, the detection of low-mass planets is more difficult and produces a detection bias on the low-mass end of the mass function (e.g. Udry & Mayor 2001; Jorissen et al. 2001). In order to reduce this bias the measurement precision must be improved by efficient and stable instrumentation and by efficient measurement and data reduction techniques.

Using the CORALIE echelle spectrograph on the 1.2-m Euler Swiss telescope at La Silla we are carrying out, since summer 1998, a large high-precision radial-velocity program (Queloz et al. 2000; Udry et al. 2000). Together with ELODIE in the northern hemisphere, CORALIE has allowed the discovery of about half of the known exoplanet candidates (Udry & Mayor 2001). Recent improvement of the data reduction software have contributed to obtain an overall instrumental precision below $\sim$3  m s^-1 (Queloz et al. 2001a) over time scales longer than 3 years. The recent asteroseismology measurements on $\alpha$ Cen A (Bouchy & Carrier 2001) have proven that the short term precision is even better, namely about 1  m s^-1 rms over 1 night. The limitations are now determined mainly by photon noise and residual astroclimatic influence.

The present paper describes the discovery of two Saturn-mass companions to the stars HD 108147 and HD 168746. HD 168746 b, with its minimum mass of only 0.77  $M_{\rm Sat}$, is one of the four sub-Saturnian planets discovered to date. On the other hand, HD 108147 b distinguishes itself by a high eccentricity, fueling the discussion of the origin of such high eccentricity in the case of short-period companions. Before the orbital parameters and the stellar characteristics of these two objects described in detail in the second part of the paper, we discuss additional improvements made recently in the data reduction, and more precisely, in the way of extracting the radial-velocity information obtained from the high-resolution spectra recorded with CORALIE.