A super-Earth and sub-Neptune transiting the late-type M dwarf LP 791-18.

Planetsoccur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planetstransiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature2960 K, and radius 0.17 R_Sun, making it the third-coolest star known to host planets. The two planets straddlethe radius gap seen for smaller exoplanets; they include a 1.1 R_Earth planeton a 0.95 day orbit and a 2.3 R_Earth planeton a 5 day orbit. Because the host star is small the loss of light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.