Radio emission in ultracool dwarfs

Previous observations of Ultracool Dwarfs at radio wavelengths opened a new window to probe the nature of their radio emission. Despite the fact that VLBI observations are still scarce in this field, they provide unique insights on the mechanisms responsible for the detected emission and, at the same time, they allow a better characterization of the orbital motion in binary or triple systems as well as their dynamical masses. In this EVN Symposium we present VLA + EVN observations of the triple system VHS 1256-1257 and LBA + AMBER/VLTI observations of the binary system AB Dor A/C. We discovered radio emission in VHS 1256-1257 at X-band spatially coincident with the central binary brown dwarf and determined the spectral index which suggests that non-thermal, optically thin, synchrotron or gyrosynchrotron radiation is the responsible mechanism. In addition, we constrained the magnetic field intensity to be of the order of a few kG. Regarding AB Dor A/C, the VLBI data clearly showed a substructure in the main star of this binary system (AB Dor A). AMBER data showed evidence for AB Dor C being not a point source but a binary system itself.