The GAPS Programme at TNG. TOI-5076b: a warm sub-Neptune planet orbiting a thick disk star in a wide binary system.

Aims. We report the confirmation of a new transiting exoplanet orbiting the star TOI-5076.Methods. We present our vetting procedure and follow-up observations which led to the confirmation of the exoplanet TOI-5076b. In particular, we employed high precision TESS photometry, high-angular resolution imaging from several telescopes and high precision radial velocities from HARPS-N.Results. From the HARPS-N spectroscopy, we determined the spectroscopic parameters of the host star: Teff=(5070143) K, log g=(4.60.3), Fe/H=(+0.200.08) and /Fe=0.050.06. The transiting planet is a warm sub-Neptune with a mass mp=(162) M, a radius rp=(3.20.1) R yielding a density p=(2.80.5) g cm3. It revolves around its star every 23.445 days.Conclusions. The host star is a metal-rich, K2V dwarf, located at about 82 pc from the Sun with a radius of R=(0.780.01) R and a mass of M=(0.800.07) M. It forms a common proper motion pair with a M-dwarf companion star located at a projected separation of 2178 au. The chemical analysis of the host-star and the Galactic space velocities indicate that TOI-5076 belongs to the old population of thick disk stars or thin-to-thick transition stars. The density of TOI-5076b suggests the presence of a large fraction by volume of volatiles overlying a massive core. We found that a circular orbit solution is only marginally favoured with respect to an eccentric orbit solution for TOI-5076b. The best-fit eccentricity for the system is e=(0.200.09).