Ground based astrometric search for extrasolar planets in stellar multiple systems

Using relative astrometry and ground based imaging, we study nearby stellar multiple systems in order to detect the astrometric signal of exoplanets around one of the stellar components. We suppress differential chromatic refraction effects by observing in the near infrared with a narrow-band filter and by choosing an iterative calibration strategy (using a stellar cluster) we obtain a precision down to 100  μ as in our relative astrometric measurements. This allows us to study the occurence and properties of exoplanets in nearby (less than 100 pc) stellar multiple systems. While trying to detect the astrometric signal of the exoplanet orbiting HD19994 A (Mayor et al. 2004), we detected the astrometric signal of a further object around the B component. This astrometric detection is confirmed by speckle interferometry and follow-up radial velocity (RV) observations. Thus, HD19994 is an exoplanet host-triple system.