A Warm Jupiter transiting an M dwarf: A TESS single-transit event confirmed with the Habitable-zone Planet Finder

We confirm the planetary nature of a warm Jupiter transiting the early M dwarf TOI-1899 using a combination of available TESS photometry; high-precision, near-infrared spectroscopy with the Habitable-zone Planet Finder; and speckle and adaptive optics imaging. The data reveal a transiting companion on an ~29 day orbit with a mass and radius of 0.66 ± 0.07 M_J and 1.15_(-0.05)^(+0.04) R_J, respectively. The star, TOI-1899, is the lowest-mass star known to host a transiting warm Jupiter, and we discuss the follow-up opportunities afforded by a warm (T_(eq) ~ 362 K) gas giant orbiting an M0 star. Our observations reveal that TOI-1899.01 is a puffy warm Jupiter, and we suggest additional transit observations to both refine the orbit and constrain the true dilution observed in TESS.