2015MNRAS.453L.103S

We analyse FORS2/VLT I-band imaging data to monitor the motions of both components in the nearest known binary brown dwarf WISE J104915.57-531906.1AB (LUH 16) over one year. The astrometry is dominated by parallax and proper motion, but with a precision of ∼ 0.2 mas per epoch we accurately measure the relative position change caused by the orbital motion of the pair. This allows us to directly measure a mass ratio of q = 0.78±0.10 for this system. We also search for the signature of a planetary-mass companion around either of the A and B component and exclude at 3σ the presence of planets with masses larger than 2MJ and orbital periods of 20-300 d. We update the parallax of LUH 16 to 500.51±0.11 mas, i.e. just within 2 pc. This study yields the first direct constraint on the mass ratio of LUH 16 and shows that the system does not harbour any close-in giant planets.