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 Mártir (México). 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<SUB>-0.12</SUB><SUP>+0.16</SUP> R<SUB>⊕</SUB> and an orbital period of 10.9 days. The outer, smaller planet has a radius of R = 1.56<SUB>-0.13</SUB><SUP>+0.15</SUP> R<SUB>⊕</SUB> 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<SUB>p</SUB> = 13.5<SUB>-9.0</SUB><SUP>+11.0</SUP> M<SUB>⊕</SUB> (&lt;36.8 M<SUB>⊕</SUB> at 2-σ) for TOI-1266 b and 2.2<SUB>-1.5</SUB><SUP>+2.0</SUP> M<SUB>⊕</SUB> (&lt;5.7 M<SUB>⊕</SUB> at 2-σ) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09<SUB>-0.05</SUB><SUP>+0.06</SUP> (&lt;0.21 at 2-σ) for TOI-1266 b and 0.04 ± 0.03 (&lt; 0.10 at 2-σ) 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.