ASTROMETRIC DETECTION OF A LOW-MASS COMPANION ORBITING THE STAR AB DORADUS

We report submilliarcsecond-precise astrometric measurements for the l ate-type star AB Doradus via a combination of VLBI (very long baseline interferometry) and HIPPARCOS data. Our astrometric analysis results in the precise determination of the kinematics of this star, which rev eals an orbital motion readily explained as caused by gravitational in teraction with a low-mass companion. From the portion of the reflex or bit covered by our data and using a revised mass of the primary star ( 0.76 M-circle dot) derived from our new value of the parallax (66.3 ma s < pi < 67.2 mas), we find the dynamical mass of the newly discovered companion to be between 0.08 and 0.11 M-circle dot. If accurate photo metric information can be obtained for the low-mass companion, our pre cise mass estimate could serve as an accurate calibration point for di fferent theoretical evolutionary models of low-mass objects. This repr esents the first detection of a low-mass stellar companion using VLBI, a technique that will become an important tool in future searches for planets and brown dwarfs orbiting other stars.