Abstract. We present the discovery of two Saturn-mass companions to HD 108147 and HD 168746. Both belong to the lightest ever discovered planets. The minimum mass of the companion to HD 168746 is of only 0.77 the mass of Saturn and its orbital period is 6.4 days. The companion to HD 108147 orbits its parent star in 10.9 days and its minimum mass is 1.34 that of Saturn. Its orbit is characterized by a high eccentricity, e = 0.50, indicating possibly the presence of a second companion. The detection of Saturn-mass planets by means of the Doppler technique demands high radial-velocity measurement precision. The two new candidates were discovered by means of the CORALIE echelle spectrograph. The instrumental accuracy of CORALIE combined with the simultaneous ThArreference technique has reached a level better than 3 m s −1 . On many observed objects the precision is now limited by photon noise. We present in this paper the weighted cross-correlation technique, which leads to an improvement in the photon noise of the computed radial velocity. We discuss as well a modification of the numerical crosscorrelation mask which reduces significantly the residual perturbation effects produced by telluric absorption lines.

Aims. It is known that stellar activity may complicate the analysis of high-precision radial-velocity spectroscopic data when looking for exoplanets signatures. We aim at quantifying the impact of stellar spots on stars with various spectral types and rotational velocities and comparing the simulations with data obtained with the HARPS spectrograph. Methods. We have developed detailed simulations of stellar spots and estimated their effects on a number of observables commonly used in the analysis of radial-velocity data when looking for extrasolar planets, such as radial-velocity curves, cross-correlation functions, bisector velocity spans, and photometric curves. Stellar and spot properties are taken into account, as well as the characteristics of the spectrograph used (generally HARPS). The computed stellar spectra are then analyzed in the same way as when searching for exoplanets. Results. 1) A first grid of simulation results (radial-velocity amplitudes, bisector velocity-span amplitudes and shapes, and photometry) is built for F-K type stars, with different stellar and spot properties. 2) It is shown quantitatively that star spots with typical sizes of 1% can mimic both radial-velocity curves and the bisector behavior of short-period giant planets around G-K type stars with a v sin i lower than the spectrograph resolution. For stars with intermediate v sin i, smaller spots may produce similar features. Such spots may complicate the search for low-mass planets on short-period orbits. In these cases, additional observables (e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the presence of short-period planets. We discuss these possibilities and show that, in some cases, photometric variations may not be enough to clearly rule out spots as explanations of the observed radial-velocity variations. This is particularly important when searching for super-Earth planets. 3) It is also stressed that quantitative values obtained for radial-velocity and bisector velocity-span amplitudes depend strongly on the detailed star properties, on the spectrograph used, on the line or set of lines used, and on the way they are measured. High-resolution spectrographs will help in distinguishing between spots and planets.

Aims. Systematic surveys to search for exoplanets have been mostly dedicated to solar-type stars sofar. We developed in 2004 a method to extend such searches to earlier A−F type dwarfs and started spectroscopic surveys to search for planets and quantify the detection limit achievable when taking the stars properties (Spectral Type, v sin i) and their actual levels of intrinsic variations into account. We give here the first results of our southern survey with HARPS. Methods. We observed 185 A-F (B − V in the range [−0.1; 0.6]) stars with HARPS and analysed them with our dedicated software. We used several criteria to probe different origins for the radial-velocity variations -stellar activity (spots, pulsations) or companions: bisector shape, radial-velocity variations amplitudes, and timescales. Results. 1) Sixty-four percent of the 170 stars with enough data points are found to be variable. Twenty are found to be binaries or candidate binaries (with stars or brown dwarfs). More than 80% of the latest type stars (once binaries are removed), are intrinsically variable at a 2 m s −1 precision level. Stars with earlier spectral type (B − V ≤ 0.2) are either variable or associated to levels of uncertainties comparable to the RV rms observed on variable stars of the same B − V. 2) We detected one long-period planetary system (presented in another paper) around an F6IV−V star. 3) We quantified the jitter due to stellar activity and we show that when taking this jitter into account in addition to the stellar parameters (spectral type, v sin i), it is still possible to detect planets with HARPS with periods of 3 days (resp. 10 days and 100 days) on 91% (resp. 83%, 61%) of them. We show that even the earliest spectral type stars are accessible to this type of search, provided they have a low projected rotational velocity and low levels of activity. 4) Taking the present data into account, we computed the actually achieved detection limits for 107 targets and discuss the limits as a function of B − V. Given the data at hand, our survey is sensitive to short-period (few days) planets and to longer ones (100 days) to a lower extent (latest type stars). We derive first constrains on the presence of planets around A−F stars for these ranges of periods.

We present the performances of a radial velocity measurement method that we developed for A-F type stars. These perfomances are evaluated through an extensive set of simulations, together with actual radial velocity observations of such stars using the ELODIE and HARPS spectrographs. We report the case of stars constant in radial velocity, the example of a binary detection on HD 48097 (an A2V star, with v sin i equal to 90 km s −1 ) and a confirmation of the existence of a 3.9 M Jup planet orbiting around HD 120136 (Tau Boo). The instability strip problem is also discussed. We show that with this method, it is in principle possible to detect planets and brown dwarfs around A-F type stars, thus allowing further study of the impact of stellar masses on planetary system formation over a wider range of stellar masses than is currently done.

Aims. We present the results of a deep imaging survey of stars surrounded by planets detected with the radial velocity technique. The purpose is to search for and to characterize long-period stellar and substellar companions. The sample contains a total of 26 stars, among which 6 exhibit additional radial velocity drifts. Methods. We used NACO, at the ESO Very Large Telescope, and PUEO-KIR, at the Candian French Hawaiian Telescope, to conduct a near-infrared coronographic survey with adaptive optics of the faint circumstellar environment of the planetary hosts. The domain investigated ranges between 0.1 to 15 (i.e. about 3 to 500 AU, according to the mean distance of the sample). The survey is sensitive to companions within the stellar and the substellar domains, depending on the distance to the central stars and on the star properties. Results. The images of 14 stars do not reveal any companions once the field objects are removed. 8 stars have close potential companions that need to be re-observed within 1-2 years to check for physical companionship. 4 stars are surrounded by faint objects which are confirmed or very probable companions. The companion to HD 13445 (Gl 86) is already known. The HD 196885 star is a new close visual binary system with a high probability of being bound. The 2 newly discovered companions, HD 1237 B and HD 27442 B, share common proper motions with the central stars. Orbital motion is detected for HD 1237 B. HD 1237 B is likely a low-mass M star, located at 70 AU (projected distance) from the primary. HD 27442 B is most probably a white dwarf companion located at about 240 AU (projected distance).