Using FORS2 on the Very Large Telescope, we have astrometrically monitored over a period of two months the two components of the brown dwarf system WISE J104915.57-531906.1, the closest one to the Sun. Our astrometric measurements - with a relative precision at the milli-arcsecond scale - allowed us to detect the orbital motion and derive more precisely the parallax of the system, leading to a distance of 2.020 ± 0.019 pc. The relative orbital motion of the two objects is found to be perturbed, which leads us to suspect the presence of a substellar companion around one of the two components. We also performed VRIz photometry of the two components and compared this with models. We confirm the flux reversal of the T dwarf.
Based on data obtained with the ESO Very Large Telescope under programme 291.C-5004.Figures 2 and 3 and Table 3 are available in electronic form at http://www.aanda.org