ε Indi Ba,Bb:: The nearest binary brown dwarf

We have carried out high angular resolution near-infrared imaging and low-resolution (R similar to 1000) spectroscopy of the nearest known brown dwarf, epsilon Indi B, using the ESO VLT NAOS /CONICA adaptive optics system. We find it to be a close binary ( as also noted by Volk et al. 2003), with an angular separation of 0.732 arcsec, corresponding to 2.65 AU at the 3.626 pc distance of the epsilon Indi system. In our discovery paper (Scholz et al. 2003), we concluded that epsilon Indi B was a similar to 50 M-Jup T2.5 dwarf: our revised finding is that the two system components (epsilon Indi Ba and epsilon Indi Bb) have spectral types of T1 and T6, respectively, and estimated masses of 47 and 28 M-Jup, respectively, assuming an age of 1.3 Gyr. Errors in the masses are +/- 10 and +/- 7 M-Jup, respectively, dominated by the uncertainty in the age determination (0.8 - 2 Gyr range). This uniquely well-characterised T dwarf binary system should prove important in the study of low-mass, cool brown dwarfs. The two components are bright and relatively well-resolved: epsilon Indi B is the only T dwarf binary in which spectra have been obtained for both components. The system has a well-established distance and age. Finally, their orbital motion can be measured on a fairly short timescale (nominal orbital period similar to 15 yrs), permitting an accurate determination of the true total system mass, helping to calibrate brown dwarf evolutionary models.