  LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two
nearly identical pre-main sequence stars of spectral type M7. The system is
remarkable among ultracool dwarfs for being the most luminous radio emitter
over the widest frequency range. Masses and luminosities are at first sight
consistent with the system being coeval at age ~80 Myr according to standard
(non-magnetic) evolutionary models. However, these models predict an average
effective temperature, 2950 +/- 5 K, which is 180 K hotter than the empirical
value. Thus, the empirical radii are oversized relative to the standard models
by ~13%. We demonstrate that magnetic stellar models can account quantitatively
for the oversizing. As a check on our models, we note that the radio emission
limits the surface magnetic field strengths: the limits depend on identifying
the radio emission mechanism. We find that the field strengths required by our
magnetic models are too strong to be consistent with gyrosynchrotron emission,
but are consistent with electron cyclotron maser emission.
