Accurate masses of very low mass stars I. Gl 570BC (0.6 M circle dot+0.4 Mcircle dot)

We present very accurate individual masses (1.2% relative accuracy) for the two components of G1 570BC, an interferometric and double-lined spectrosco pic binary system. They were obtained from new high accuracy radial velocit y and angular separation measurements, analysed together with previously pu blished measurements. From those data we determine a much improved orbit th rough a simultaneous least square fit to the radial velocity, visual, and p arallax information. The derived masses and absolute magnitudes generally v alidate the theoretical and empirical mass-luminosity relations around 0.5 M., but point rewards some low level discrepancies at the 0.1 to 0.15 magni tude level. Forthcoming results of this observing program will extend the c omparison to much lower masses with similar accuracy.