Multi-Epoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass
  Measurements and Confirmation of Radio Emission from Both Components
  Surveys have shown that up to one tenth of all ultracool dwarfs (UCDs) are
appreciable radio emitters, with their emission attributed to a combination of
gyrosynchrotron radiation and the electron cyclotron maser instability (ECMI).
2M J0746+2000AB is a close stellar binary comprised of an L0 and L1.5 dwarf
that was previously identified as a source of 5 GHz radio emission. We used
very-long-baseline interferometry (VLBI) to precisely track the radio emission
over seven epochs in 2010-2017, and found both components to be radio emitters
-- the first such system identified -- with the secondary component as the
dominant source of emission in all epochs. The previously identified 2.07 h
periodic bursts were confirmed to originate from the secondary component,
although an isolated burst was also identified from the primary component. We
additionally fitted the VLBI absolute astrometric positions jointly with
existing relative orbital astrometry derived from optical/IR observations with
Markov-chain Monte Carlo (MCMC) methods to determine the orbital parameters of
the two components. We found the masses of the primary and secondary optical
components to be 0.0795 +/- 0.0003 Msun and 0.0756 +/- 0.0003 Msun,
respectively, representing the most precise mass estimates of any UCDs to date.
Finally, we place a 3-sigma upper limit of 0.9 Mjup au on the mass and
separation of planets orbiting either of the two components.
