An extremely low-mass companion to an F0-type star in a total-eclipsing close binary

ABSTRACT M-type dwarfs orbiting stars earlier than G-type have been reported to exhibit larger radii than those predicted by stellar models. To explain the inconsistencies between observations and models, more precise mass and radius measurements of M-type stars in such kind of binaries are needed to refine evolutionary models. By using photometric data from Transiting Exoplanet Survey Satellite and spectroscopic observations from Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST), we conducted a study on the binary system TIC 431124333, finding TIC 431124333 is a total-eclipsing detached binary where an F0-type primary is accompanied by an M-type secondary (F+M) with a short period of 1.88 d. The photometric analysis with the Wilson–Devinney (W-D) code yields a mass ratio of 0.13$\,\pm\, 0.2$. Meanwhile, the mass ratio derived from single-lined radial velocities based on medium-resolution spectra from LAMOST is approximately 0.15, which demonstrates TIC 431124333, as a totally eclipsing binary, can deliver a reliable mass ratio with photometric data. The companion has a mass of 0.16 (2) ${\rm M}_{\odot }$ and radius of 0.21 (1)$R_{\odot }$. Given the exceptionally low mass ratio, the secondary component of TIC 431124333 is highly likely to be formed via disc fragmentation and currently in the pre-main-sequence stage. Collective samples of F+M binaries suggest their M-type companions may still be undergoing gravitational contraction towards the main sequence, in contrast to single M-type stars or low-mass binaries exhibiting radius inflation. Further testing of this hypothesis requires additional systems with precisely determined masses and radii.