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Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs (BDs) are poorly understood. Themultiplicity properties and minimummass of the BD 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 0.025M☉ and 0.034M☉, and projected separations of 0.31 AU and 0.19 AU, making them the lowestmass and tightest field BD binaries known. The discovery of a population of such binaries indicates that BD binaries can robustly form at least down to masses of ~0.02M☉. Future microlensing surveys will measure a mass-selected sample of BD binary systems, which can then be directly compared to similar samples of stellar binaries. [ABSTRACT FROM AUTHOR]

We present the analysis of the gravitational microlensing event OGLE-2013-BLG-0102. The light curve of the event is characterized by a strong short-term anomaly superposed on a smoothly varying lensing curve with a moderate magnification Amax ~1.5. It is found that the event was produced by a binary lens with a mass ratio between the components of q = 0.13 and the anomaly was caused by the passage of the source trajectory over a caustic located away from the barycenter of the binary. From the analysis of the effects on the light curve due to the finite size of the source and the parallactic motion of the Earth, the physical parameters of the lens system are determined. The measured masses of the lens components are M1 = 0.096 ± 0.013 M⊙ and M2 = 0.012±0.002 M⊙, which correspond to near the hydrogen-burning and deuterium-burning mass limits, respectively. The distance to the lens is 3.04±0.31 kpc and the projected separation between the lens components is 0.80 ± 0.08 AU. [ABSTRACT FROM AUTHOR]