TESS's "Coolest" M dwarf PC: Validating TOI-904, a multiplanet system including a cold sub-Neptune

We report the discovery of two sub-Neptunes transiting the early M dwarf star, TOI 904 (TIC 261257684). Both planets were first detected in observations from the Transiting Exoplanet Survey Satellite (TESS). Reconnaissance radial velocity measurements (taken with SMARTS/CHIRON and EULER/CORALIE) and high resolution speckle imaging with adaptive optics (obtained from SOAR/HRCAM and ZORRO/Gemini South) show no evidence of an eclipsing binary or a nearby companion, validating the planetary nature of these candidates. We measure the orbital period and radius of the known TOI-904.01 to be 10.9 days and 2.62REarth, respectively. We also find a colder outer planet, TIC 261257684.02, with a similar radius (~2.6REarth) and a period of 84 days. This outer planet is the longest-period M dwarf exoplanet found by TESS, with an estimated equilibrium temperature ~219K. As the three other validated planets with comparable host stars and orbital periods were observed around dim stars (Jmag>12) by Kepler, this planet, orbiting a brighter star (Jmag =9.6), could be the coldest M dwarf planet accessible for atmospheric follow-up