  The nearest known binary brown dwarf WISE J104915.57-531906.1AB (LUH 16) is a
well-studied benchmark for our understanding of substellar objects. Previously
published astrometry of LUH 16 obtained with FORS2 on the Very Large Telescope
was affected by errors that limited its use in combination with other datasets,
thereby hampering the determination of its accurate orbital parameters and
masses. We improve upon the calibration and analysis of the FORS2 astrometry
with the help of Gaia DR2 to generate a high-precision dataset that can be
combined with present and future LUH 16 astrometry. We demonstrate its use by
combining it with available measurements from the Hubble Space Telescope (HST)
and Gemini/GeMS and deriving updated orbital and mass parameters. Using Gaia
DR2 as astrometric reference field, we derived the absolute proper motion and
updated the absolute parallax of the binary to 501.557 +/- 0.082 mas. We
refined the individual dynamical masses of LUH 16 to 33.5 +/- 0.3 M[Jupiter]
(component A) and 28.6 +/- 0.3 M[Jupiter] (component B), which corresponds to a
relative precision of ~1% and is three to four times more precise than previous
estimates. We found that these masses show a weak dependence on one datapoint
extracted from a photographic plate from 1984. The exact determination of a
residual mass bias, if any, will be possible when more high-precision data can
be incorporated in the analysis.
