HD 21520 b: a warm sub-Neptune transiting a bright G dwarf

We report the discovery and validation of HD 21520 b, a transiting planet found with TESS and orbiting a bright G dwarf (V=9. 2, T₄₅₅ = 5871 62 K, R_ = 1. 04 0. 02\, R_). HD 21520 b was originally alerted as a system (TOI-4320) consisting of two planet candidates with periods of 703. 6 and 46. 4 days. However, our analysis supports instead a single-planet system with an orbital period of 25. 12920. 0001 days and radius of 2. 70 0. 09\, R_. Three full transits in sectors 4, 30 and 31 match this period and have transit depths and durations in agreement with each other, as does a partial transit in sector 3. We also observe transits using CHEOPS and LCOGT. SOAR and Gemini high-resolution imaging do not indicate the presence of any nearby companions, and MINERVA-Australis and CORALIE radial velocities rule out an on-target spectroscopic binary. Additionally, we use ESPRESSO radial velocities to obtain a tentative mass measurement of 7. 9^+3. 2-₃. ₀\, M_, with a 3- upper limit of 17. 7 M_. Due to the bright nature of its host and likely significant gas envelope of the planet, HD 21520 b is a promising candidate for further mass measurements and for atmospheric characterization.