Abstract

We report the discovery of four transiting F-M binary systems with companions between 0.1 and 0.2 M-circle dot 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-circle dot, 0.147(-0.004)(+0.003) R-circle dot), HATS551-019B (0.17(-0.01)(+0.01) M-circle dot, 0.18(-0.01)(+0.01) R-circle dot), HATS551-021B (0.132(-0.005)(+0.014) M-circle dot, 0.154(-0.008)(+0.006) R-circle dot and HATS553-001B (0.20(-0.02)(+0.01) M-circle dot, 0.22(-0.01)(+0.01) R-circle dot). 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.