TESS discovery of a sub-Neptune orbiting a mid-M dwarf TOI-2136

We present the discovery of TOI-2136 b, a sub-Neptune planet transiting a nearby M4.5V-type star every 7.85 d, identified through photometric measurements from the Transiting Exoplanet Survey Satellite (TESS) mission. The host star is located 33 pc away with a radius of R-* = 0.34 +/- 0.02 R-circle dot, a mass of 0.34 +/- 0.02 M-circle dot, and an effective temperature of 3342 +/- 100 K. We estimate its stellar rotation period to be 75 +/- 5 d based on archival long-term photometry. We confirm and characterize the planet based on a series of ground-based multiwavelength photometry, high-angular-resolution imaging observations, and precise radial velocities from Canada-France-Hawaii Telescope (CFHT)/SpectroPolarimetre InfraROUge (SPIRou). Our joint analysis reveals that the planet has a radius of 2.20 +/- 0.17 R-circle plus and a mass of 6.4 +/- 2.4 M-circle plus. The mass and radius of TOI-2136 b are consistent with a broad range of compositions, from water-ice to gas-dominated worlds. TOI-2136 b falls close to the radius valley for M dwarfs predicted by thermally driven atmospheric mass-loss models, making it an interesting target for future studies of its interior structure and atmospheric properties.