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 M\'artir (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⊕ and an orbital period of 10.9 days. The
outer, smaller planet has a radius of R=1.56−0.13+0.15 R⊕ 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
Mp = 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 K 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