  Context: We report the discovery of TOI-519 b (TIC 218795833), a transiting
substellar object (R = 1.07 RJup) orbiting a faint M dwarf (V = 17.35) on a
1.26 d orbit. Brown dwarfs and massive planets orbiting M dwarfs on
short-period orbits are rare, but more have already been discovered than
expected from planet formation models. TOI-519 is a valuable addition into this
group of unlikely systems, and adds towards our understanding of the boundaries
of planet formation.
  Aims: We set out to determine the nature of the Transiting Exoplanet Survey
Satellite (TESS ) object of interest TOI-519 b.
  Methods: Our analysis uses a SPOC-pipeline TESS light curve from Sector 7,
multicolour transit photometry observed with MuSCAT2 and MuSCAT, and transit
photometry observed with the LCOGT telescopes. We estimate the radius of the
transiting object using multicolour transit modelling, and set upper limits for
its mass, effective temperature, and Bond albedo using a phase curve model that
includes Doppler boosting, ellipsoidal variations, thermal emission, and
reflected light components.
  Results: TOI-519 b is a substellar object with a radius posterior median of
1.07 RJup and 5th and 95th percentiles of 0.66 and 1.20 RJup, respectively,
where most of the uncertainty comes from the uncertainty in the stellar radius.
The phase curve analysis sets an upper effective temperature limit of 1800 K,
an upper Bond albedo limit of 0.49, and a companion mass upper limit of 14
MJup. The companion radius estimate combined with the Teff and mass limits
suggests that the companion is more likely a planet than a brown dwarf, but a
brown-dwarf scenario is more likely a priori given the lack of known massive
planets in 1 day orbits around M dwarfs with Teff < 3800 K, and the existence
of some (but few) brown dwarfs.
