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

Cone search capability for table J/MNRAS/437/2831/stars (List of studied stars)