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We report the discovery of two TESS sub-Neptunes orbiting the early M dwarfTOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initiallyobserved in TESS sector 12 with twin sizes of 2.49R$_\oplus$ and2.31R$_\oplus$, respectively. Through observations in five additional sectorsin the TESS primary mission and the first and second extended missions, theorbital periods of both planets were measured to be 10.887$\pm$0.001 and83.999$\pm$0.001 days, respectively. Reconnaissance radial velocitymeasurements (taken with EULER/CORALIE) and high resolution speckle imagingwith adaptive optics (obtained from SOAR/HRCAM and Gemini South/ZORRO) show noevidence of an eclipsing binary or a nearby companion, which together with thelow false positive probabilities calculated with the statistical validationsoftware TRICERATOPS establish the planetary nature of these candidates. Theouter planet, TOI-904 c, is the longest-period M dwarf exoplanet found by TESS,with an estimated equilibrium temperature of 217K. As the three other validatedplanets with comparable host stars and orbital periods were observed by Kepleraround much dimmer stars (J$_{mag}$ $>$ 12), TOI-904 c, orbiting a brighterstar (J$_{mag}$ $=$ 9.6), is the coldest M dwarf planet easily accessible foratmospheric follow-up. Future mass measurements and transmission spectroscopyof the similar sized planets in this system could determine whether they arealso similar in density and composition, suggesting a common formation pathway,or whether they have distinct origins.

Separated twins or just siblings? A multi-planet system around an M dwarf including a cool sub-Neptune