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Description
We present the discovery of a transiting mini-Neptune around TOI-1201, a relatively bright and moderately young early M dwarf (J~9.5mag, ~600-800Myr) in an equal-mass ~8arcsecond-wide binary system, using data from the Transiting Exoplanet Survey Satellite (TESS), along with follow-up transit observations. With an orbital period of 2.49d, TOI-1201~b is a warm mini-Neptune with a radius of R_b_=2.415+/-0.090R_{Earth}_. This signal is also present in the precise radial velocity measurements from CARMENES, confirming the existence of the planet and providing a planetary mass of M_b_=6.28+/-0.88M_{Earth}_ and, thus, an estimated bulk density of 2.45^+0.48^_-0.42_g/cm^3^. The spectroscopic observations additionally show evidence of a signal with a period of 19d and a long periodic variation of undetermined origin. In combination with ground-based photometric monitoring from WASP-South and ASAS-SN, we attribute the 19d signal to the stellar rotation period (P_rot_=19-23d), although we cannot rule out that the variation seen in photometry belongs to the visually close binary companion. We calculate precise stellar parameters for both TOI-1201 and its companion. The transiting planet is an excellent target for atmosphere characterization (the transmission spectroscopy metric is 97^+21^_-16_) with the upcoming James Webb Space Telescope. It is also feasible to measure its spin-orbit alignment via the Rossiter-McLaughlin effect using current state-of-the-art spectrographs with submeter per second radial velocity precision.
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