Searching for GEMS: discovery and characterization of two brown dwarfs around M dwarfs

Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( 68.0_−2.2^+2.2M_J ) and TOI-5610b ( 40.4_−1.0^+1.0M_J ), two moderately massive brown dwarfs orbiting early M-dwarf hosts (T_eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are period P = 10.40046 ± 0.00002 days, radius R_BD = 0.824_−0.031^+0.033 R_J, and low eccentricity e = 0.0962_−0.0046^+0.0027 . In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios of q = 0.150. For TOI-5610b, the best-fitting parameters are period P = 7.95346 ± 0.00002 days, radius R_BD = 0.887_−0.031^+0.031 R_J, and moderate eccentricity e = 0.354_−0.012^+0.011. Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the Johnson K band. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods (P < 13 days) exhibit a dearth of 13 M_J < M_BD < 40 M_J companions (q < 0.1) compared to those at slightly wider separations.