The mass-radius relationship for very low mass stars: Four new discoveries from the HATSouth Survey

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^+0.005 _-0.006M⊙, 0.147^+0.003 _-0.004 R⊙), HATS551-019B (0.17^+0.01 _-0.01M⊙, 0.18^+0.01 _-0.01 R⊙), HATS551-021B (0.132^+0.014 _-0.005M⊙, 0.154^+0.006 _-0.008 R⊙) and HATS553-001B (0.20^+0.01 _-0.02M⊙, 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 per cent systematic deviation between the measured radii and theoretical isochrone models.