  We report the discovery of a brown dwarf orbiting a M1 host star. We first
identified the brown dwarf within the Next Generation Transit Survey data, with
supporting observations found in TESS sectors 11 and 38. We confirmed the
discovery with follow-up photometry from the South African Astronomical
Observatory, SPECULOOS-S, and TRAPPIST-S, and radial velocity measurements from
HARPS, which allowed us to characterise the system. We find an orbital period
of ~1.25 d, a mass of 69.0+5.3-4.8 MJ, close to the Hydrogen burning limit, and
a radius of 0.95 +- 0.05 RJ. We determine the age to be >0.5 Gyr, using model
isochrones, which is found to be in agreement with SED fitting within errors.
NGTS-28Ab is one of the shortest period systems found within the brown dwarf
desert, as well as one of the highest mass brown dwarfs that transits an M
dwarf. This makes NGTS-28Ab another important discovery within this scarcely
populated region.
