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 a $G=15.35$ mag M3 dwarf star. HATS-71 is the coolest M dwarf 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 space-based 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 $1.024\pm 0.018$ ${R}_{{\rm{J}}}$ and mass of $0.37\,\pm 0.24$ ${M}_{{\rm{J}}}$ (95% confidence upper limit of $\lt 0.80$ ${M}_{{\rm{J}}}$ ), while the star has a mass of $0.4861\pm 0.0060$ ${M}_{\odot }$ and a radius of $0.4783\pm 0.0060$ ${R}_{\odot }$ .