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Extrasolar planet searches targeting very low-mass stars and brown dwarfs arehampered by intrinsic or instrumental limitations. Time series of astrometricmeasurements with precisions better than one milli-arcsecond can yield newevidence on the planet occurrence around these objects. We present firstresults of an astrometric search for planets around 20 nearby dwarf stars withspectral types M8-L2. Over a timespan of two years, we obtained I-band imagesof the target fields with the FORS2 camera at the Very Large Telescope. Usingbackground stars as references, we monitored the targets' astrometrictrajectories, which allowed us to measure parallax and proper motions, setlimits on the presence of planets, and to discover the orbital motions of twobinary systems. We determined trigonometric parallaxes with an average accuracyof 0.09 mas (~0.2 %) resulting in a reference sample for the study of ultracooldwarfs at the M/L transition, whose members are located at distances of 9.5-40pc. This sample contains two newly discovered tight binaries (DE0630-18 andDE0823-49) and one previously known wide binary (DE1520-44). Only one targetshows I-band variability >5 mmag r.m.s. We derived planet exclusion limits thatset an upper limit of 9 % to the occurrence of giant planets with masses >5MJup in intermediate-separation (0.01-0.8 AU) orbits around M8-L2 dwarfs. Wedemonstrated that astrometric observations with an accuracy of 120 micro-arcsecover two years are feasible from the ground and can be used for a planet searchsurvey. The detection of two tight very low-mass binaries showed that oursearch strategy is efficient and may lead to the detection of planetary-masscompanions through follow-up observations.

Astrometric planet search around southern ultracool dwarfs I: First results, including parallaxes of 20 M8-L2 dwarfs