Abstract
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
−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.