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Using astrometric measurements obtained with the FORS2/VLT camera, we aresearching for low-mass companions around 20 nearby ultracool dwarfs. With asingle-measurement precision of 0.1 milli-arcseconds, our survey is sensitiveto a wide range of companion masses from planetary companions to binarysystems. Here, we report the discovery and orbit characterisation of a newultracool binary at a distance of 19.5 pc from Earth that is composed of theM8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-onorbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, whichcorresponds to a relative separation in semimajor axis of approximately 1.1 AU.We obtained a high-resolution optical spectrum with UVES/VLT and measured thesystem's heliocentric radial velocity. The spectrum does not exhibit lithiumabsorption at 670.8 nm, indicating that the system is not extremely young. Apreliminary estimate of the binary's physical parameters tells us that it iscomposed of a primary at the stellar-substellar limit and a massive brown-dwarfcompanion. DE0630-18 is a new very low-mass binary system with awell-characterised orbit.

Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18