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Accurate masses of very low mass stars: I Gl 570BC (0.6+0.4 Msol)

We present very accurate individual masses (1.2% relative accuracy) for the two components of Gl 570BC, an interferometric and double-lined spectroscopic binary system. They were obtained from new high accuracy radial velocity and angular separation measurements, analysed together with previously published measurements. From those data we determine a much improved orbit through a simultaneous least square fit to the radial velocity, visual, and parallax information. The derived masses and absolute magnitudes generally validate the theoretical and empirical mass-luminosity relations around 0.5 Msol, but point towards some low level discrepancies at the 0.1 to 0.15 magnitude level. Forthcoming results of this observing program will extend the comparison to much lower masses with similar accuracy.