TOI-6884b: A low-mass brown dwarf transiting a slightly evolved star

We report the discovery of a low-mass transiting brown dwarf orbiting TOI-6884 (TIC 156514476, Tmag = 11.4) from NASA’s Transiting Exoplanet Survey Satellite (TESS) mission. The TESS light curves initially suggested an orbital period of ~14.42 days; however, our high-precision ground-based radial velocity measurements and multi-epoch time-series photometry reveal this to be a harmonic alias. We determine the true orbital period to be 4.808264+0.000015-0.000014 days and confirm the substellar nature of the companion. TOI-6884b has a mass of 26.32+0.98-0.93MJ, a radius of 0.927+0.51-0.52 RJ, and resides on a nearly circular orbit (e=0.067+0.010-0.012). Its host star is a late F-type slightly evolved star with M_ = 1.410+0.075-0.069 M_, R_ = 1.840+0.072-0.073 R_, log g = 4.057+0.045-0.039, [ Fe/H] = 0.094+0.073-0.068 dex, and Teff=6330⁺¹⁸⁰₋₁₆₀ K. TOI-6884b is a key addition to the small population of well-characterized transiting brown dwarfs orbiting host stars that have left the main sequence. The detection of such systems will contribute to our understanding of dynamical histories and structural evolution of short-period substellar companions around evolved stars.