  Although many models have been proposed, the physical mechanisms responsible
for the formation of low-mass brown dwarfs are poorly understood. The
multiplicity properties and minimum mass of the brown-dwarf mass function
provide critical empirical diagnostics of these mechanisms. We present the
discovery via gravitational microlensing of two very low-mass, very tight
binary systems. These binaries have directly and precisely measured total
system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31
AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf
binaries known. The discovery of a population of such binaries indicates that
brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun.
Future microlensing surveys will measure a mass-selected sample of brown-dwarf
binary systems, which can then be directly compared to similar samples of
stellar binaries.
