TOI-6508 b: A massive transiting brown dwarf orbiting a low-mass star

We report the discovery of a transiting brown dwarf orbiting a low-mass star, TOI-6508 b. Today, only ∼50 transiting brown dwarfs have been discovered. TOI-6508 b was first detected with data from the Transiting Exoplanet Survey Satellite (TESS) in Sectors 10, 37 and 63. Ground-based follow-up photometric data were collected with the SPECULOOS-South (Search for habitable Planets EClipsing ULtra-cOOl Stars) and LCOGT-1m telescopes, and RV measurements were obtained with the Near InfraRed Planet Searcher (NIRPS) spectrograph. We find that TOI-6508 b has a mass of Mp = 72.5−5.1+7.6 MJup and a radius of Rp = 1.03 ± 0.03 RJup. Our modeling shows that the data are consistent with an eccentric orbit of 19 day and an eccentricity of e = 0.28−0.08+0.09. TOI-6508 b has a mass ratio of MBD/M★ = 0.40, makes it the second highest mass ratio brown dwarf that transits a low-mass star. The host has a mass of M★ = 0.174 ± 0.004 M⊙, a radius of R★ = 0.205 ± 0.006 R⊙, an effective temperature of Teff = 2930 ± 70 K, and a metallicity of [Fe/H] = −0.22 ± 0.08. This makes TOI-6508 b an interesting discovery that has come to light in a region still sparsely populated.