Copyright © 2021, Oxford University PressOsborn, Hugh P.Armstrong, D. J.Adibekyan, V.Collins, K. A.Delgado Mena, E.Howell, Steve B.Hellier, C.King, G. W.Lillo Box, J.Nielsen, L. D.Otegi, J. F.Santos, N. C.Ziegler, C.Anderson, D. R.Briceño, C.Burke, C. J.Bayliss, D.Barrado, D.Bryant, E. M.Brown, D. J. A.Barros, S. C. C.Bouchy, F.Caldwell, D. A.Conti, D.Díaz, R. F.Dragomir, D.Deleuil, M.Demanegon, O. D. S.Dorn, C.Daylan, T.Figueira, P.Helled, R.Hoyer, S.Jenkins, J. S.Jensen, E. L. N.Latham, D. W.Law, N.Louie, D.Mann, A. W.Osborn, A.Pollacco, D.Rodríguez, D. R.Rackham, B. V.Ricker, G.Scott, N. J.Sousa, S. G.Seager, S.Stassun, K. G.Smith, J. C.Strom, P.Udry, S.Villaseñor, J. N.Vanderspek, R.West, R.Wheatley, P. J.Winn, J. N.Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-07372022-02-152022-02-152021-01-25Monthly Notices of the Royal Astronomical Society 502(4): 4842–4857(2021)0035-8711https://academic.oup.com/mnras/article-abstract/502/4/4842/6119926http://hdl.handle.net/20.500.12666/589We report the discovery of HD 110113 b (TESS object of interest-755.01), a transiting mini-Neptune exoplanet on a 2.5-d orbit around the solar-analogue HD 110113 (Teff = 5730 K). Using TESS photometry and High Accuracy Radial velocity Planet Searcher (HARPS) radial velocities gathered by the NCORES program, we find that 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 on to a substantial (0.1–1 per cent) 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, HD 110113 c, which has a mass of 10.5 ± 1.2  M⊕ and a period of 6.744+0.008−0.009  d.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttps://creativecommons.org/licenses/by-nc-nd/4.0/Planets and satellites: detectionStars: individual: HD110113info:eu-repo/semantics/article10.1093/mnras/stab1821365-2966http://dx.doi.org/10.13039/501100001871http://dx.doi.org/10.13039/501100000271http://dx.doi.org/10.13039/501100011033http://dx.doi.org/10.13039/100000104info:eu-repo/semantics/openAccess