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 ( \pi _ { abs } = 57.975 \pm 0.045 mas , corresponding to a distance of d = 17.249 \pm 0.013 pc ) , proper motion ( \mu _ { \alpha cos \delta } = -247.99 \pm 0.10 mas yr ^ { -1 } , \mu _ { \delta } = -183.58 \pm 0.22 mas yr ^ { -1 } ) , and orbital motion is obtained .
Despite the fact that the two components have nearly identical masses to within \pm 2 % , the secondary ’ s radio emission exceeds that of the primary by a factor of \gtrsim 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 to 10 M _ { jup } with orbital periods of 600 to 10 days , respectively .
Additionally , we use the full orbital solution of the binary to derive an upper limit for the semi-major 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 .