Astrometric companions: low-mass stars, brown-dwarfs and planets

High precision astrometry at the ≈10 uas level is now routinely achieved from the ground with the Very Long BaseIine Interferometry (VLBI) technique. The milli-arcsecond (mas) angular resolution of VLBI instruments allows to resolve binary systems were angular separations can be as small as a few mas. In addition, high precision astrometry allows searching for unseen companions, by the detection of the astrometric signature or “wobble” due to the gravitational pull of the companion. We have used astrometry at radio wavelengths to search for giant planets around very low mass stars, which are difficult to find by other techniques. In this contribution, we present the properties of low-mass young binaries and of two giant planets discovered with this technique.