  TOI-1899 b is a rare exoplanet, a temperate Warm Jupiter orbiting an M-dwarf,
first discovered by Ca\~nas et al. (2020) from a TESS single-transit event.
Using new radial velocities (RVs) from the precision RV spectrographs HPF and
NEID, along with additional TESS photometry and ground-based transit follow-up,
we are able to derive a much more precise orbital period of $P =
29.090312_{-0.000035}^{+0.000036}$ d, along with a radius of $R_p = 0.99 \pm
0.03~R_J$. We have also improved the constraints on planet mass, $M_p = 0.67
\pm 0.04~M_J$, and eccentricity, which is consistent with a circular orbit at
2$\sigma$ ($e = 0.044_{-0.027}^{+0.029}$). TOI-1899 b occupies a unique region
of parameter space as the coolest known ($T_{eq} \approx$ 380 K) Jovian-sized
transiting planet around an M-dwarf; we show that it has great potential to
provide clues regarding the formation and migration mechanisms of these rare
gas giants through transmission spectroscopy with JWST as well as studies of
tidal evolution.

Earth and Planetary Astrophysics