With fourteen epochs of fringe tracking data spanning 1.7y from Fine Guidance Sensor 3 we have obtained a parallax ( \pi _ { abs } = 113.1 \pm 0.3 mas ) and perturbation orbit for Gl 791.2A . Contemporaneous fringe scanning observations yield only three clear detections of the secondary on both interferometer axes . They provide a mean component magnitude difference , \Delta V = 3.27 \pm 0.10 . The period ( P = 1.4731 yr ) from the perturbation orbit and the semi-major axis ( a = 0.963 \pm 0.007 AU ) from the measured component separations with our parallax provide a total system mass { \cal M } _ { A } + { \cal M } _ { B } = 0.412 \pm 0.009 { \cal M } _ { \sun } . Component masses are { \cal M } _ { A } = 0.286 \pm 0.006 M _ { \sun } and { \cal M } _ { B } = 0.126 \pm 0.003 { \cal M } _ { \sun } . Gl 791.2A and B are placed in a sparsely populated region of the lower main sequence mass-luminosity relation where they help define the relation because the masses have been determined to high accuracy , with errors of only 2 % .