1 Introduction

We report in this paper on our radial-velocity measurements of HD 80606, the primary star of the visual binary system HD 80606-HD 80607. These observations reveal the presence of a 3.9 Jovian-mass planet (minimum mass) in a very eccentric orbit around this solar-type star.

The variable velocity of HD 80606 was first noticed by the G-Dwarf Planet Search (Latham 2000), a reconnaissance of nearly 1000 nearby G dwarfs that uses the HIRES high-resolution spectrograph (Vogt et al. 1994) mounted on the 10-m Keck 1 telescope at the W. M. Keck Observatory (Hawaii, USA) to identify extrasolar planet candidates. The star was then followed up by the ELODIE Planet Search Survey team (Mayor & Queloz 1996; Udry et al. 2000) using the ELODIE fiber-fed echelle spectrograph (Baranne et al. 1996) mounted on the Cassegrain focus of the 1.93-m telescope at the Observatoire de Haute-Provence (CNRS, France).

The ELODIE velocities are obtained by cross-correlating the observed spectra with a numerical template. The instrumental drifts are monitored and corrected using the "simultaneous thorium-argon technique" with dual fibers (Baranne et al. 1996). The achieved precision with this instrument is of the order of 10 m s$^{\rm -1}$. The HIRES instrumental profile and drifts are monitored using an iodine gas absorption cell (Marcy & Butler 1992). The radial velocities are derived from the spectra using the TODCOR code (Zucker & Mazeh 1994), a two-dimensional correlation algorithm.

The observations of HD 80606 started in April 1999 with HIRES. With a velocity difference of 267 m s$^{\rm -1}$ in less than one month between the first two measurements, the variability of this source was quickly detected. In July 1999, we started an ELODIE radial-velocity follow up of 6 non-active slow-rotating radial-velocity variable stars detected with HIRES, including HD 80606. The first ELODIE measurement for this star was obtained during our November 1999 run. The discovery of the planetary companion orbiting HD 80606 has been recently announced together with 10 other new extrasolar planet candidates (April 4th 2001 ESO PR). Among these is the planetary companion to HD 178911 B (Zucker et al., in prep.), another one of the candidates identified with HIRES.

The stellar characteristics of the two components of the HD 80606-HD 80607 visual binary are presented in Sect. 2. The radial-velocity data and the orbital solution are presented in Sect. 3. The very high orbital eccentricity is discussed in Sect. 4. The 61 radial-velocity measurements presented in Sect. 3 as well as the iron line list we used in Sect. 2 will be made available in electronic form at the CDS in Strasbourg via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/L27