The mass function and statistics of binaries provide important diagnostics of the star formation process .
Despite this importance , the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects .
Here we report the microlensing discovery and characterization of a binary lens composed of very low-mass stars just above the hydrogen-burning limit .
From the combined measurements of the Einstein radius and microlens parallax , we measure the masses of the binary components of 0.10 \pm 0.01 M _ { \odot } and 0.09 \pm 0.01 M _ { \odot } .
This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population .