NGTS-38 photometry and spectroscopy

We present the discovery of TIC-65910228 b / NGTS-38 b, a giant exoplanet with a radius of 1.081+/-0.047 RJ and a mass of 4.78^+0.39^_-0.37_MJ on a long period (180.52797+/-0.00036 day), moderately eccentric (e=0.3086+/-0.010) orbit transiting a bright (V=10.230+/-0.020 mag) metal rich ([Fe/H]=0.33+/-0.09dex) F6V-F7V type host star. The planet was initially detected from a single transit in TESS Sector 33. A photometric monitoring campaign of 228 nights with NGTS detected a transit egress of the planet, which together with spectroscopic radial velocity monitoring with CORALIE and HARPS identified an orbital period of ~180.5d.

These radial velocity measurements also showed the mass of the companion to be planetary. Additional transit observations coordinated by the TESS follow-up observing program allowed further confirmation and refinement of this period. With its relatively cool equilibrium temperature of 457+/-11K, NGTS-38 b joins a small but growing population of well characterised transiting warm-Jupiters and has one of the longest periods of any discovered to date.