A Super-Earth And A Sub-Neptune Orbiting The Bright, Quiet M3 Dwarf Toi-1266

We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (K = 8.8), quiet, and nearby (37 pc) 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)(+0.16) R-circle plus and an orbital period of 10.9 days. The outer, smaller planet has a radius of R = 1.56(-0.13)(+0.15) R-circle plus 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)(+11.0) M-circle plus (<36.8 M-circle plus at 2-sigma) for TOI-1266 b and 2.2(-1.5)(+2.0) M-circle plus (<5.7 M-circle plus at 2-sigma) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09(-0.05)(+0.06) (<0.21 at 2-sigma) for TOI-1266 b and 0.04 +/- 0.03 (<0.10 at 2-sigma) for TOI-1266 c. The equilibrium temperatures of both planets are of 413 +/- 20 and 344 +/- 16 K, 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.