We report the discovery of a hot (Teq ≈ 1055 K) planet in the small-planet radius valley that transits the Sun-like star TOI-733. It was discovered as part of the KESPRINT follow-up program of TESS planets carried out with the HARPS spectrograph. TESS photometry from sectors 9 and 36 yields an orbital period of {P{{orb}}} = 4.884765 - 2.4e - 5 + 1.9e - 5 days and a radius of {R{p}} = 1.992 - 0.090 + 0.085 R⊕. Multi-dimensional Gaussian process modelling of the radial velocity measurements from HARPS and activity indicators gives a semi-amplitude of K = 2.23 ± 0.26 m s-1, translating into a planet mass of {M{p}} = 5.72 - 0.68 + 0.70 M⊕. These parameters imply that the planet is of moderate density ({ρ {p}} = 3.98 - 0.66 + 0.77 g cm-3) and place it in the transition region between rocky and volatile-rich planets with H/He-dominated envelopes on the mass-radius diagram. Combining these with stellar parameters and abundances, we calculated planet interior and atmosphere models, which in turn suggest that TOI-733 b has a volatile-enriched, most likely secondary outer envelope, and may represent a highly irradiated ocean world. This is one of only a few such planets around G-type stars that are well characterised.

Full Table A.1 is only available at the CDS via anonymous cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/674/A117