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Large sub-Neptunes are uncommon around the coolest stars in the Galaxy andare rarer still around those that are metal-poor. However, owing to the largeplanet-to-star radius ratio, these planets are highly suitable for atmosphericstudy via transmission spectroscopy in the infrared, such as with JWST. Here wereport the discovery and validation of a sub-Neptune orbiting the thick-disk,mid-M dwarf star TOI-2406. We first infer properties of the host star byanalysing the star's near-infrared spectrum, spectral energy distribution, andGaia parallax. We use multi-band photometry to confirm that the transit eventis on-target and achromatic, and we statistically validate the TESS signal as atransiting exoplanet. We then determine physical properties of the planetthrough global transit modelling of the TESS and ground-based time-series data.We determine the host to be a metal-poor M4V star, located at a distance of 56pc, with a sub-solar metallicity $(\mathrm{[Fe/H] = -0.38 \pm 0.07})$, and amember of the thick disk. The planet is a relatively large sub-Neptune for theM-dwarf planet population, with $\mathrm{R_p = 2.94 \pm 0.17}\mathrm{R_\oplus}$ and $\mathrm{P = 3.077}$ d, producing transits of 2% depth.We note the orbit has a non-zero eccentricity to 3$\mathrm{\sigma}$, promptingquestions about the dynamical history of the system. This system is aninteresting outcome of planet formation and presents a benchmark forlarge-planet formation around metal-poor, low-mass stars. The system warrantsfurther study, in particular radial velocity follow-up to determine the planetmass and constrain possible bound companions. Furthermore, TOI-2406 b is a goodtarget for future atmospheric study through transmission spectroscopy,particularly in the category of warm sub-Neptunes.

A large sub-Neptune transiting the thick-disk M4V TOI-2406