# The Mass-Radius Relationship for Very Low Mass Stars: Four New Discoveries from the HATSouth Survey

## TL;DR

This paper reports four new very low mass star companions discovered by HATSouth, providing precise mass and radius measurements, and discusses their implications for stellar models and the radius anomaly in fully-convective stars.

## Contribution

The study presents four new low-mass star discoveries with detailed characterization, expanding the sample for testing stellar models and examining the radius anomaly.

## Key findings

- Measured radii are systematically larger than models by about 5%.
- The sample supports the existence of the radius anomaly in fully-convective stars.
- Provides precise mass and radius data for very low mass stars.

## Abstract

We report the discovery of four transiting F-M binary systems with companions between 0.1-0.2 Msun in mass by the HATSouth survey. These systems have been characterised 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 synchronisation. These new very low mass companions are HATS550-016B (0.110 -0.006/+0.005 Msun, 0.147 -0.004/+0.003 Rsun), HATS551-019B (0.17 -0.01/+0.01 Msun, 0.18 -0.01/+0.01 Rsun), HATS551-021B (0.132 -0.005/+0.014 Msun, 0.154 -0.008/+0.006 Rsun), HATS553-001B (0.20 -0.02/+0.01 Msun, 0.22 -0.01/+0.01 Rsun). 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% systematic deviation between the measured radii and theoretical isochrone models.