2007A&A...464..635B

Because of the lack of precise masses, the coverage of the main-sequence empirical mass-luminosity relation for stars in the mass range from 0.6M_☉ to 0.9M_☉ is incomplete. The nearby K-type visual and spectroscopic binary GJ765.2=MLR224 is a good candidate for new reliable points in this significant part of the relation. We have found a combined orbital solution for the pair and derived physical properties of the components using interferometric and spectroscopic data. The diffraction-limited speckle observations were mostly collected at the 6m BTA telescope, and the velocities of the components were obtained using the CORAVEL radial velocity scanner on the Swiss 1m telescope. In a combined solution, the orbital period is found to be 11.919yr. The masses of the GJ 765.2 components are M_A=0.831±0.020M_☉ and M_B=0.763±0.019M_☉. The obtained orbital parallax of the system, π_orb=31.0±0.5mas, is 7 percent lower than the Hipparcos value. The absolute V magnitudes of the stars, derived from the measured speckle magnitude differences, are: M_V^A=5.99±0.04 and M_V^B=6.64±0.05. The effective temperatures of the components, T_eff^A=5060±130K and T_eff^B=4690±160K, follow from the V-K and J-K color indices. The star metallicity value, estimated from the 6 m telescope spectrum, is [M/H]=-0.35±0.15dex. The presented individual masses have 2.4% and 2.5% relative accuracies. Therefore, the components of GJ 765.2 rank among a dozen stars with masses accurate to within a few percent in the mass range 0.6-0.9M_☉. The existing data on the kinematics of GJ765.2 and its chromospheric activity indicate that the binary belongs to the middle age (3-4x10⁹yr) thin disk population of the galaxy.