A 43 d transiting Neptune and two 25 d Saturns from TESS, NGTS, and ASTEP

Beyond orbital periods of 10 d, there is a dearth of known transiting gas giants. On longer orbits, planets are less affected by their host star, and become ideal probes of planet formation, migration, and evolution. We report the discovery of a longperiod Neptune and two Saturns, each initially identified as single transits in the TESS (Transiting Exoplanet Survey Satellite) photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS (Next Generation Transit Survey) and ASTEP (Antarctic Search for Transiting ExoPlanets). High-resolution radial velocity mass measurements using CORALIE and HARPS (High Accuracy Radial Velocity Planet Searcher) confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of 43.12655^+0.00012_{−0.00017 d}, radius of 3.65 ± 0.22 R_⊕, and mass of 19.1^+4.9_−4.5 M_⊕ for NGTS-34 b, making it one of the longest period well-characterized transiting Neptunes. Orbiting a late F-type star, bright in the K band (Kmag ≅ 7.9), it is amenable for cool atmosphere studies using James Webb Space Telescope or Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large). TOI-4940 b is a small Saturn on a 25.867811^+0.000058_−0.000056 d orbit with a radius of 6.61 ± 0.37 R_⊕ and an upper mass limit < 89 M_⊕. NGTS-35 b(=TOI-6669 b) is a larger Saturn on a 25.241192 ± 0.000022 d, moderately eccentric orbit (e = 0.192^+0.037_−0.033), with a radius of 10.90 ± 0.65 R_⊕ and a mass of 152⁺²²₋₁₉ M_⊕. With an assumed albedo A = 0.3, each of these planets has an equilibrium temperature below 700 K, with NGTS-35 b especially cold at 450 K. These three giants add to the small but growing population of long-period planets that can further our understanding of planet formation mechanisms.