Planetary systems in close binary stars: the case of HD196885 Combined astrometric and radial velocity study

Received; accepted Context. More than fifty candidate planets are presently known to orbit one component of a binary or a multiple star system. Planets can therefore form and survive in such an environment, although recent observing surveys indicate that short-separation binaries do not favour the presence of a planetary system around one of the component. Dynamical interactions with the secondary component can actually significantly impact the giant planet formation and evolution. For this reason, rare close binaries hosting giant planets offer an ideal laboratory to explore the properties and the stability of such extreme planetary systems. Aims. In the course of our CFHT and VLT coronographic imaging survey dedicated to the search for faint companions of exoplanet host stars, a close ( ∼ 20 AU) secondary stellar companion to the exoplanet host HD196885 A was discovered. In this study, our aim is to monitor the orbital motion of the binary companion. Combining radial velocity and high contrast imaging observations, we aim to derive the orbital properties of the complete system and to test its dynamical stability to reveal its formation. Methods. For more than 4 years, we have used the NaCo near-infrared adaptive optics instrument to monitor the astrometric position of HD 196885 B relative to A. The system was observed at five different epochs from August 200