N ov 2 00 8 Dynamical masses for the nearest brown dwarf binary : ε Indi Ba

We present preliminary astrometric results for the closest known brown dwarf binary to the Sun:ε Indi Ba, Bb at a distance of 3.626 pc. Via ongoing monitoring o f the relative separation of the two brown dwarfs (spectral types T1 and T6) with the VLT NACO near-IR adaptive optics system since June 2004, we obtain a model-independent dynam ical total mass for the system of 121 MJup, some 60% larger than the one obtained by McCaughrean et al. ( 2004), implying that the system may be as old as 5 Gyr. We have also been monitoring the a bsolute astrometric motions of the system using the VLT FORS2 optical imager since August 2005 t o de ermine the individual masses. We predict a periastron passage in early 2010, by which time t he system mass will be constrained to <1 MJup and we will be able to determine the individual masses accura tely in a dynamical, modelindependent manner.