We present multi-epoch astrometric radio observations with the Very Long Baseline Array (VLBA) of the young ultracool-dwarf binary LSPM J1314 1320AB. The radio emission comes from the secondary star. Combining the VLBA data with Keck near-infrared adaptive-optics observations of both components, a full astrometric fit of parallax (p abs = 57.975 ± 0.045 mas, corresponding to a distance of d = 17.249 ± 0.013 pc), proper motion (µ acos d = -247.99 ± 0.10 mas yr-1, µ d = -183.58 ± 0.22 mas yr-1), and orbital motion is obtained. Despite the fact that the two components have nearly identical masses to within ±2%, the secondary’s radio emission exceeds that of the primary by a factor of ?30, suggesting a difference in stellar rotation history, which could result in different magnetic field configurations. Alternatively, the emission could be anisotropic and beamed toward us for the secondary but not for the primary. Using only reflex motion, we exclude planets of mass 0.7-10 M jup with orbital periods of 600-10 days, respectively. Additionally, we use the full orbital solution of the binary to derive an upper limit for the semimajor axis of 0.23 au for stable planetary orbits within this system. These limits cover a parameter space that is inaccessible with, and complementary to, near-infrared radial velocity surveys of ultracool dwarfs. Our absolute astrometry will constitute an important test for the astrometric calibration of Gaia.
