  The astrometric space mission Gaia is expected to detect a large number of
brown dwarf binary systems with close orbits and determine astrometric orbit
solutions. This will provide key information for the formation and evolution of
brown dwarfs, such as the binary frequency and dynamical masses. Known brown
dwarf binaries with orbit constraints from other techniques will play an
important role. We are carrying out one of the most precise and long-lasting
radial velocity surveys for brown dwarf binaries in the Cha I star-forming
region at the VLT. We were able to add two orbit determinations to the small
group of a handful of brown dwarf and very low-mass binaries with characterized
RV orbits. We show here that the astrometric motion of both systems can be
detected with Gaia. We predict an astrometric signal of about 1.2 - 1.6
milliarcseconds (mas) for the brown dwarf binary ChaHa8 and of 0.4 - 0.8 mas
for the very low-mass binary CHXR74. We take the luminosity of the companion
into account for these estimates and present a relation for the astrometric
signature of a companion with non-negligible luminosity.
