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 ($68.0^{ +2.2}_{ -2.2} \ \mj$) and TOI-5610b ($40.4^{ +1.0}_{ -1.0} \ \mj$), two moderately massive brown dwarfs orbiting early M dwarf hosts ($\teff = 3569 \pm 59 \ K$ and $3618 \pm 59 \ K$, respectively). For TOI-5389Ab, the best fitting parameters are period $P=10.40046 \pm 0.00002$ days, radius $R_{ \rm BD}=0.824^{ +0.033}_{ -0.031}$~\rj, and low eccentricity $e=0.0962^{ +0.0027}_{ -0.0046}$. 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 $P=7.95346 \pm 0.00002$ days, radius $R_{ \rm BD}=0.887^{ +0.031}_{ -0.031}$ \rj, and moderate eccentricity $e=0.354^{ +0.011}_{ -0.012}$. Both targets are expected to have shallow but potentially observable secondary transits: $\lesssim 500$ ppm in Johnson K band for both. 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 $13 \mj < M_{ \rm BD} < 40 \mj$ companions ($q$ $<$ 0.1) compared to those at slightly wider separations.
Categories: astro-ph.EP astro-ph.SR