The CORALE survey for Southern extra-solar planets - IV. Intrinsic stellarlimitations to planet searches with radial-velocity techniques

Activity related phenomena can induce radial-velocity variations, which can be very important when dealing with extra-solar planet search programmes r equiring high-precision radial-velocity measurements. In this paper we present a new chromospheric activity index, S-COR, based o n the Ca II H line central reemission, and constructed using CORALIE spectr a. After one year of measurements, values of SCOR are available for a sub-s ample of stars of the Geneva extra-solar planet search programme. After tra nsforming the SCOR values into the Mount-Wilson "S" scale we obtained value s of the Ca II H and K flux corrected from photospheric emission (R'(HK)) f or the stars. The first results are presented, and in particular we focus o n the study of the relation between the observed radial-velocity scatter an d the chromospheric activity index R'(HK), for F, G and K dwarfs.