2014MNRAS.437.2831Z

We report the discovery of four transiting F-M binary systems with companions between 0.1 and 0.2M_☉ in mass by the HATSouth survey. These systems have been characterized via a global analysis of the HATSouth discovery data, combined with high-resolution radial velocities and accurate transit photometry observations. We determined the masses and radii of the component stars using a combination of two methods: isochrone fitting of spectroscopic primary star parameters and equating spectroscopic primary star rotation velocity with spin-orbit synchronization. These new very low mass companions are HATS550-016B (0.110_-0.006^+0.005 M_☉, 0.147_-0.004^+0.003 R_☉), HATS551-019B (0.17_-0.01^+0.01 M_☉, 0.18_-0.01^+0.01 R_☉), HATS551-021B (0.132_-0.005^+0.014 M_☉, 0.154_-0.008^+0.006 R_☉) and HATS553-001B (0.20_-0.02^+0.01 M_☉, 0.22_-0.01^+0.01 R_☉). We examine our sample in the context of the radius anomaly for fully convective low-mass stars. Combining our sample with the 13 other well-studied very low mass stars, we find a tentative 5 percent systematic deviation between the measured radii and theoretical isochrone models.