Starspot Coverage on Two New K-type Low-mass Eclipsing Binaries with Radius Inflation

Abstract Utilizing data from the Transiting Exoplanet Survey Satellite (TESS) and LAMOST, we present a photometric and spectroscopic investigation of two new K-type low-mass eclipsing binaries, TIC 56913729 and TIC 97729372. Our analysis yields masses and radii for TIC 56913729, M 1 = 0.7822 ± 0.0054 M ⊙ , R 1 = 0.7891 ± 0.0021 R ⊙ , and M 2 = 0.7532 ± 0.0052 M ⊙ , R 2 = 0.7648 ± 0.0021 R ⊙ . For TIC 97729372, the results are M 1 = 0.6410 ± 0.0058 M ⊙ , R 1 = 0.6537 ± 0.0069 R ⊙ and M 2 = 0.6480 ± 0.0058 M ⊙ , R 2 = 0.6418 ± 0.0062 R ⊙ . In addition, by analyzing the out-of-eclipse starspot light variations, the lower limit of starspot coverage varies in different TESS sectors from 2% to 12%. We observed a clear radius inflation in the mass–radius diagram for both stars, when plotted against PARSEC and SPOT isochrones with 1 Gyr. Yet, this apparent discrepancy disappears when the comparison is made with either an older PARSEC isochrone (12 Gyr) or a SPOT isochrone (250 Myr) with high starspot coverage ( F spot ∼ 85%), which is significantly higher than the typical starspot coverage deduced from light curves. Due to the lack of strong age constraints, we cannot firmly exclude that the observed radius inflation may be the result of a post-main-sequence evolutionary effect, although the spectral and kinematic properties of these stars are hardly compatible with 10–12 Myr old (Pop II) stars. It is more likely that the radius inflation is produced by the strong magnetic activity in these rapidly rotating stars, even if it is impossible to infer the actual total spot coverage, due to the age uncertainties.