  Extrasolar planet searches targeting very low-mass stars and brown dwarfs are
hampered by intrinsic or instrumental limitations. Time series of astrometric
measurements with precisions better than one milli-arcsecond can yield new
evidence on the planet occurrence around these objects. We present first
results of an astrometric search for planets around 20 nearby dwarf stars with
spectral types M8-L2. Over a timespan of two years, we obtained I-band images
of the target fields with the FORS2 camera at the Very Large Telescope. Using
background stars as references, we monitored the targets' astrometric
trajectories, which allowed us to measure parallax and proper motions, set
limits on the presence of planets, and to discover the orbital motions of two
binary systems. We determined trigonometric parallaxes with an average accuracy
of 0.09 mas (~0.2 %) resulting in a reference sample for the study of ultracool
dwarfs at the M/L transition, whose members are located at distances of 9.5-40
pc. This sample contains two newly discovered tight binaries (DE0630-18 and
DE0823-49) and one previously known wide binary (DE1520-44). Only one target
shows I-band variability >5 mmag r.m.s. We derived planet exclusion limits that
set an upper limit of 9 % to the occurrence of giant planets with masses >5
MJup in intermediate-separation (0.01-0.8 AU) orbits around M8-L2 dwarfs. We
demonstrated that astrometric observations with an accuracy of 120 micro-arcsec
over two years are feasible from the ground and can be used for a planet search
survey. The detection of two tight very low-mass binaries showed that our
search strategy is efficient and may lead to the detection of planetary-mass
companions through follow-up observations.
