We report the discovery of HD 110113 b (TOI-755.01), a transiting mini-Neptune exoplanet on a 2.5-day orbit around the solar-analogue HD 110113. Using TESS photometry and HARPS radial velocities gathered by the NCORES program, we find HD 110113 b has a radius of 2.05±0.12 R⊕ and a mass of 4.55±0.62 M⊕. The resulting density of 2.90+0.75−0.59 g/cm^3 is significantly lower than would be expected from a pure-rock world; therefore, HD 110113 b must be a mini-Neptune with a significant volatile atmosphere. The high incident flux places it within the so-called radius valley; however, HD 110113 b was able to hold onto a substantial (0.1-1\%) H-He atmosphere over its ∼4 Gyr lifetime. Through a novel simultaneous gaussian process fit to multiple activity indicators, we were also able to fit for the strong stellar rotation signal with period 20.8±1.2 d from the RVs and confirm an additional non-transiting planet with a mass of 10.5±1.2 M⊕ and a period of 6.744+0.008−0.009 d

{"references": ["Osborn et al (2021). 10.1093/mnras/stab182"]}