Invited review abstract

Binary brown dwarf systems provide crucial benchmarks for testing the low-mass end of evolutionary models as both components will have the same age and chemical composition. $\epsilon$ Indi Ba, Bb, (T1 and T6), are the closest known brown dwarfs to Earth (3.6224 pc). Moreover, with a K4.5 star companion, $\epsilon$ Indi A, allows the break of the substellar mass-age degeneracy. Our observations using the ESO VLT include relative and absolute astrometric monitoring and high angular resolution optical, near-infrared, and thermal-infrared imaging and medium-resolution spectroscopy. Using our spectroscopic observations and VRIzJHKL'M' broad-band photometry of the individual components we derived luminosities of log = -4.699 $\pm$ 0.017 and -5.232 $\pm$ 0.020, effective temperatures of 1300 - 1340 K and 880 - 940 K and surface gravities of log g = 5.25 and 5.50 for $\epsilon$ Indi Ba and Bb respectively. The relative orbital motion of the brown dwarfs has been monitored since June 2004 with the VLT NACO near-IR adaptive optics system determining a total dynamical system mass of 121 $\pm$ 1 M$_{\rm{Jup}}$, significantly in excess of previous estimates. Combining our system mass determination and derived luminosity, evolutionary models predict an age of 3.7 - 4.3 Gyr, also significantly higher than previous estimates. We have also been monitoring the absolute astrometric motion of the system since August 2005 against a network of field stars using the VLT FORS2 optical imager and we will present the individual masses of $\epsilon$ Indi Ba, Bb, which assuming they are coeval will be able to test the mass-luminosity relation for intermediate age brown dwarfs without the usual ambiguity due to age. This system will allow us to test the predictions of evolutionary and atmospheric models with a precision never obtained before, and give a tightly constrained benchmark that the next generation of models must be able to reproduce.