The GAPS programme at TNG

The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs; P ≲ 10 d, R p > 8 R ⊕ ) 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 inner-orbit companions (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 orb = 3.63 d, R p = 3.15 R ⊕ ) and an outer hot giant planet (TOI-1533 c, P orb = 8.06 d, R p > 7.5 R ⊕ ) with substantial H/He by mass (ρ p < 0.48 g cm −3 ), both transiting an active K-dwarf star ( T eff ≈ 5130 K; V (mag) ≈ 11). Our joint modelling of stellar activity and planetary signals from radial velocities (HARPS-N) and transits (TESS) allowed us to detect the planetary Keplerian signals (approximately 10 σ) and 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 b / M c about 0.8), together with the super-Neptune mass of the large outer companion ( M c ≈ 40 M ⊕ ), makes this system unusual compared to the other known HJs with low-mass inner companions.