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Abstract. The G0 V star HD 166435 has been observed by the fiber-fed spectrograph ELODIE as one of the targets in the large extra-solar planet survey that we are conducting at the Observatory of Haute-Provence. We detected coherent, low-amplitude, radial-velocity variations with a period of 3.7987 days, suggesting a possible close-in planetary companion. Subsequently, we initiated a series of high-precision photometric observations to search for possible planetary transits and an additional series of Ca II H and K observations to measure the level of surface magnetic activity and to look for possible rotational modulation. Surprisingly, we found the star to be photometrically variable and magnetically active. A detailed study of the phase stability of the radial-velocity signal revealed that the radial-velocity variability remains coherent only for durations of about 30 days. Analysis of the time variation of the spectroscopic line profiles using line bisectors revealed a correlation between radial velocity and line-bisector orientation. All of these observations, along with a one-quarter cycle phase shift between the photometric and the radial-velocity variations, are well explained by the presence of dark photospheric spots on HD 166435. We conclude that the radial-velocity variations are not due to gravitational interaction with an orbiting planet but, instead, originate from line-profile changes stemming from star spots on the surface of the star. The quasi-coherence of the radial-velocity signal over more than two years, which allowed a fair fit with a binary model, makes the stability of this star unusual among other active stars. It suggests a stable magnetic field orientation where spots are always generated at about the same location on the surface of the star.

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Planets have been detected orbiting several solar-type stars using high-precision radial velocity (v r) measurements. While changes in v r can be measured with an accuracy of a few m s ?1 , there has been relatively little study of how other astrophysical processes, such as magnetic activity, may e ect the observed velocities. In this paper, we use published data and simple models to explore the contributions to v r from two activity-related sources, starspots and convective inhomogeneities, as these features rotate across the disk and evolve in time. Radial velocity perturbations due to both of these sources increase with rotation and the level of surface activity. Our models indicate that for solar-age G stars, the amplitude of perturbations due to spots is A S < 5 m s ?1 , increasing to A S 30 to 50 m s ?1 for Hyades-age G stars. If f S is the starspot area coverage, we nd A S / f 0:9 S v sin i. The e ects of convective inhomogeneities (as observed in line bisector variations) appear to depend on both rotation and spectral type. Young (active) F and G dwarfs can have convective v r perturbations with amplitudes A C > 50 m s ?1 , while v r amplitudes are reduced for stars with lower v sin i and cooler T e. We show that v r data from the literature display similar trends with v sin i and T e. A S and A C will be strongest at or near timescales related to magnetic activity variations: rotation, active region growth and decay, and activity cycles. Thus, knowledge of these timescales and typical A S and A C values are important in searching for extra-solar planets, especially those around younger, more active stars or those with small v r re ex amplitudes (i.e., < 20 m s ?1). We discuss implications of our results for current planet detections and planet search strategies.

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