We present very accurate orbital parameters and mass measurements ( 2.4 % accuracy ) for the well known very low mass triple system Gl 866 . We obtain first orbital elements for the short-period orbit and greatly improve the long period orbit . All three stars have masses close to 0.1 M _ { \mathord { \odot } } , and the system thus provides the strongest constraints to date on the mass-luminosity relation close to the brown dwarf limit .