Fundamental effective temperature measurements for eclipsing binary stars – VIII. NIRPS spectroscopy of CD–27 2812

Abstract There are very few M-dwarfs with accurate independent measurements of their mass, radius and effective temperature (Teff) that can be used to test stellar models for these low-mass stars. We aim to use high-resolution, near-infrared spectroscopy to measure the mass of M-dwarfs in eclipsing binary systems with solar-type stars and to measure the flux ratio between the two stars at near-infrared wavelengths. This information can then be combined with the analysis of the light curve, photometry, and the parallax to measure the mass, radius and Teff for both stars. We have used the TESS light curve and spectra observed with the HARPS and NIRPS spectrographs to measure the following model-independent radii and masses for CD−27 2812, an F9 V star in an eclipsing binary with a much fainter M dwarf companion on a short near-circular orbit (P=7.8 d) : R1 = 1.721 ± 0.004R⊙, R2 = 0.531 ± 0.002R⊙, M1 = 1.3597 ± 0.0024M⊙, and M2 = 0.5624 ± 0.0006M⊙ We show how the NIRPS spectra can be used to measure the flux ratio in the J and H bands. This information, combined with published photometry and the Gaia DR3 parallax, leads to the following effective temperature measurements: Teff, 1 = 6197 ± 55 K, Teff, 2 = 3770 ± 28 K. This study demonstrates that it is now feasible to use eclipsing binaries to accurately measure Teff for M-dwarf stars for which we also have independent mass and radius measurements.