The GAPS programme at TNG: LXXVI. TOI-1533: a compact system hosting a super-Neptune-mass pair with disparate radii

The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs, P $\lesssim$ 10 d, $R_{\mathrm{p}}>8R_{\oplus }$) has long been a matter of debate. While most of them are found to be ``lonely’', there is a rare population of HJs hosting small companions on inner orbits (eight known as of May 2026). Their peculiar architecture suggests a gentle disc-migration mechanism. In this study, we present the discovery and characterisation of the multi-planet system TOI-1533, comprising an inner sub-Neptune (TOI-1533 b, $P_{\mathrm{o}\mathrm{r}\mathrm{b}}=3.63$ d, $R_{\mathrm{p}}=3.15R_{\oplus }$) and an outer hot giant planet (TOI-1533 c, $P_{\mathrm{o}\mathrm{r}\mathrm{b}}=8.06$ d, $R_{\mathrm{p}}>7.5R_{\oplus }$) with substantial H/He by mass (${\rho }_{\mathrm{p}}<0.48$ g cm${}^{-3}$), both transiting an active K-dwarf star ($T_{\mathrm{e}\mathrm{f}\mathrm{f}}\approx$ 5130 K; $V$ (mag) $\approx$ 11). Our joint modelling of stellar activity and planetary signals from radial velocities (HARPS-N) and transits (TESS) allows us to detect their Keplerian signals (approximately $10\sigma$) and to isolate the stellar modulation. The inclusion of simultaneous photometry in the multi-dimensional Gaussian processes formalism was a fundamental addition to the spectroscopic activity indicators, enabling the disentanglement of stellar activity from planetary signals. The mass ratio of the two confirmed planets ($M_{\mathrm{b}}\mathrm{/}{M}_{\mathrm{c}}$ about 0.8), together with the super-Neptune mass of the large outer companion ($M_{\mathrm{c}}\approx 40M_{\oplus }$), makes this system unusual compared to the other few HJs with low-mass inner companions.