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We report the discovery and characterization of two transiting planets aroundthe bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun)detected during Sector 10 observations of the Transiting Exoplanet SurveySatellite (TESS). Combining the TESS photometry with HARPS radial velocities,as well as ground-based follow-up transit observations from MEarth and LCOGTtelescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, aradius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet,TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34d, associated with the rotational period of the star. The analysis of fifteenyears of ground-based photometric monitoring data and the inspection ofdifferent spectral line indicators confirm this assumption. The bulk densitiesof TOI-776 b and c allow for a wide range of possible interior and atmosphericcompositions. However, both planets have retained a significant atmosphere,with slightly different envelope mass fractions. Thanks to their location nearthe radius gap for M dwarfs, we can start to explore the mechanism(s)responsible for the radius valley emergence around low-mass stars as comparedto solar-like stars. While a larger sample of well-characterized planets inthis parameter space is still needed to draw firm conclusions, we tentativelyestimate that the stellar mass below which thermally-driven mass loss is nolonger the main formation pathway for sculpting the radius valley is between0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system isalso an excellent target for the James Webb Space Telescope, providing aremarkable laboratory to break the degeneracy in planetary interior models andto test formation and evolution theories of small planets around low-massstars.

A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776