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

Beyond orbital periods of 10 days, 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 long period Neptune and two Saturns, each initially identified as single transits in the TESS photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS and ASTEP. High-resolution radial velocity mass measurements using CORALIE and HARPS confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of $43.12655_{-0.00017}^{+0.00012}~$days, radius of 3.65 ± 0.22 R⊕, and mass of $19.1_{-4.5}^{+4.9}~\mathrm{M_{\rm{\oplus }}}$ 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 JWST or Ariel. TOI-4940 b is a small Saturn on a $25.867811_{-0.000056}^{+0.000058}~$day 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 day, moderately eccentric orbit ($e = 0.192_{-0.033}^{+0.037}$), with a radius of 10.90 ± 0.65 R⊕ and a mass of $152_{-19}^{+22}~\mathrm{M_{\rm{\oplus }}}$. With an assumed albedo A = 0.3, each of these planets has an equilibrium temperature below 700K, 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.