An Ultra-Short-Period Transiting Super-Earth Orbiting The M3 Dwarf Toi-1685

Dynamical histories of planetary systems, as well as the atmospheric evolution of highly irradiated planets, can be studied by characterizing the ultra-short-period planet population, which the TESS mission is particularly well suited to discover. Here, we report on the follow-up of a transit signal detected in the TESS sector 19 photometric time series of the M3.0 V star TOI-1685 (2MASS J04342248+4302148). We confirm the planetary nature of the transit signal, which has a period of P-b =0.6691403(-0.0000021)(+0.0000023) d, using precise radial velocity measurements taken with the CARMENES spectrograph. From the joint photometry and radial velocity analysis, we estimate the following parameters for TOI-1685 b: a mass of M-b = 3.78(-0.63)(+0.63) a radius of R-b = 1.70(-0.07)(+0.07) R-circle plus, which together result in a bulk density of rho(b) = 4.21(-0.82)(+0.95) g cm(-3), and an equilibrium temperature of T-eq = 1069(-16)(+16) K. TOI-1685 b is the least dense ultra-short-period planet around an M dwarf known to date. TOI-1685 b is also one of the hottest transiting super-Earth planets with accurate dynamical mass measurements, which makes it a particularly attractive target for thermal emission spectroscopy. Additionally, we report with moderate evidence an additional non-transiting planet candidate in the system, TOI-1685 [c], which has an orbital period of P-c =9.02(-0.12)(+0.10) d.