HATS-71b: A Giant Planet Transiting an M3 Dwarf Star in TESS Sector 1

We report the discovery of HATS-71b, a transiting gas giant planet on a P = 3.7955 day orbit around aG = 15.35 mag M3 dwarf star. HATS-71 is the coolest Mdwarf star known to host a hot Jupiter. The loss of light during transits is 4.7%, more than in any other confirmed transiting planet system. The planet was identified as a candidate by the ground-based HATSouth transit survey. It was confirmed using ground-based photometry, spectroscopy, and imaging, as well as spacebased photometry from the NASA Transiting Exoplanet Survey Satellite mission (TIC 234523599). Combining all of these data, and utilizing Gaia.DR2, we find that the planet has a radius of 1.024 +/- 0.018 R-J and mass of 0.37 +/- 0.24 M-J (95% confidence upper limit of <0.80 M-J), while the star has a mass of 0.4861 +/- 0.0060 M-circle dot and a radius of 0.4783 +/- 0.0060 R-circle dot.