Precision measurement of a brown dwarf mass in a binary system in the
  microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035
  Context. Brown dwarfs are poorly understood transition objects between stars
and planets, with several competing mechanisms having been proposed for their
formation. Mass measurements are generally difficult for isolated objects but
also for brown dwarfs orbiting low-mass stars, which are often too faint for
spectroscopic follow-up. Aims. Microlensing provides an alternative tool for
the discovery and investigation of such faint systems. Here we present the
analysis of the microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035, which
is due to a binary system composed of a brown dwarf orbiting a red dwarf.
Methods. Thanks to extensive ground observations and the availability of space
observations from Spitzer, it has been possible to obtain accurate estimates of
all microlensing parameters, including parallax, source radius and orbital
motion of the binary lens. Results. After accurate modeling, we find that the
lens is composed of a red dwarf with mass $M_1 = 0.149 \pm 0.010M_\odot$ and a
brown dwarf with mass $M_2 = 0.0463 \pm 0.0031M_\odot$, at a projected
separation of $a_\perp = 0.585$ au. The system has a peculiar velocity that is
typical of old metal-poor populations in the thick disk. Percent precision in
the mass measurement of brown dwarfs has been achieved only in a few
microlensing events up to now, but will likely become common with the Roman
space telescope.
