Interferometric Astrometry of the Low-Mass Binary G[CLC]l[/CLC] 791.2 (= HU Del) Using [ITAL]HUBBLE SPACE TELESCOPE[/ITAL] Fine Guidance Sensor 3: Parallax and Component Masses

With fourteen epochs of fringe-tracking data spanning 1.7 yr from Fine Guidance Sensor 3 we have obtained a parallax (π abs = 113.1 ± 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, Δ V = 3.27 ± 0.10. The period ( P = 1.4731 yr), from the perturbation orbit, and the semimajor axis ( a = 0.963 ± 0.007 AU), from the measured component separations with our parallax, provide a total system mass M A + M B = 0.412 ± 0.009 M ⊙ . Component masses are M A = 0.286 ± 0.006 M ⊙ and M B = 0.126 ± 0.003 M ⊙ . Gl 791.2A and Gl 791.2B 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%.