2020A&A...642A..49D

We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (K=8.8), quiet, and nearby (37pc) M3V dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Martir (Mexico). We also include additional ground-based follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner, larger planet has a radius of R=2.37_–0.12^+0.16R_⊕ and an orbital period of 10.9 days. The outer, smaller planet has a radius of R=1.56_–0.13^+0.15R_⊕ on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of M_p=13.5_–9.0^+11.0M_⊕ (<36.8M_⊕ at 2-σ) for TOI-1266 b and 2.2_–1.5^+2.0M_⊕ (<5.7M_⊕ at 2-σ) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09_–0.05^+0.06 (<0.21 at 2-σ) for TOI-1266 b and 0.04±0.03 (<0.10 at 2-σ) for TOI-1266 c. The equilibrium temperatures of both planets are of 413±20 and 344±16K, respectively, assuming a null Bond albedo and uniform heat redistribution from the day-side to the night-side hemisphere. The host brightness and negligible activity combined with the planetary system architecture and favourable planet-to-star radii ratios makes TOI-1266 an exquisite system for a detailed characterisation.