A four-planet system orbiting the old thick disk star TOI-1203

TOI-1203 is a bright (V, =, 8. 6) G3, V star known to host a transiting warm sub-Neptune on a 25. 5-day orbit. Here we report on an intensive high-precision radial velocity and photometric follow-up campaign carried out with the spectrograph and the ̧heops space telescope. We found that TOI-1203 has an effective temperature of K, a mass of M_⋆, =, 0. 886, ±, 0. 036 M_⊙, a radius of R_⋆, =, 1. 179, ±, 0. 011 R_⊙, and an enhancement of α elements relative to iron of α/Fe =, 0. 21, ±, 0. 04. With an age of ∼12. 5 Gyr, TOI-1203 belongs to the old, α-element enhanced stellar population of the galactic thick disk. We spectroscopically confirmed the planetary nature of the 25. 5-day sub-Neptune TOI-1203 d, measured its mass (M_ and refined its radius (R_ We discovered the presence of an additional transiting super-Earth on a 4. 2-day orbit (TOI-1203 b) with a mass of M_ and a radius of R_ We also revealed the presence of two additional low-mass planets at 13. 1 d and 204. 6 d (TOI-1203 c and e), with minimum masses of M_ c sin, i c and M_ e sin, i e We found that the outer planet TOI-1203 e lies on an eccentric orbit with e_ We performed a stability analysis of the system confirming that there are configurations consistent with the observed parameters that are dynamically stable over billion-year timescales. While analyzing the time series, we discovered that the full width at half maximum of the cross-correlation function shows a significant long-period signal (∼615 d) that has no counterpart in the radial velocity data or in the remaining ancillary time series. We significantly detected the same signal in the full width at half maximum of the Th-Ar calibration lines used to compute the nightly wavelength solution, and attributed this systematic effect to a long-term variation of the instrumental profile.