TOI-1201 b: a mini-Neptune transiting a bright and moderately young M dwarf

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 Satel- lite, 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 . 090 R ⊕ . 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 . 88 M ⊕ and, thus, an estimated bulk density of 2 . 45 + 0 . 48 − 0 . 42 gcm − 3 . The spectroscopic observations additionally show evidence of a signal with a period of 19d and a long periodic variation of unde-termined 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.