The GAPS programme at TNG LXX. HD128717 B/Gaia-6 B: A long-period eccentric low-mass brown dwarf from astrometry and radial velocities

Context. The transition regime between giant planets (GPs) and brown dwarfs (BDs) is still an open subject of study in exoplanetary science. A complete understanding of the population of long-period GPs and BDs would be pivotal for improving our knowledge of this topic, but the number of such objects with precisely measured orbital and physical parameters remains small. Moreover, their dynamical influence on smaller companions in inner orbits is still unclear. Aims. Within the framework of the Global Architecture of Planetary Systems collaboration (GAPS), we aim to confirm and characterise sub-stellar companion candidates from Gaia DR3, and to study the potential presence of additional lower mass planets in their systems. Methods. We present the results of an intensive high-precision radial velocity (RV) monitoring of HD 128717, which hosts the astrometric candidate Gaia-ASOI-009. We used the HARPS-N spectrograph at Telescopio Nazionale Galileo (TNG) to collect a high- cadence RV time series of the target. We used Markov chain Monte Carlo (MCMC) analyses to refine the Gaia DR3 orbital solution of the companion and, finally, performed a combined model of RV and proper motion anomaly (PMa) to derive the complete 3D orbit of the companion. We also ran a suite of numerical simulations to confirm our results. Results. We confirmed the sub-stellar nature of Gaia-ASOI-009 (i.e. Gaia-6 B). From the combined RV+PMa fit, we confirm that it is a high-eccentricity low-mass BD with P-B = 9.37(-0.05)(+0.06), M-B = 19.8 +/- 0.5 M-J, and e(B) = 0.85, i(B) = 130 degrees. The derived orbital solution differs significantly from the one published in Gaia DR3. Through a series of dedicated simulations, we demonstrated that this discrepancy arises from a degeneracy in the Gaia DR3 astrometric solution. Specifically, the combination of Gaia-6 B long orbital period and high eccentricity, both poorly constrained by the limited time span of DR3, led to an incorrect solution characterised by a shorter period and lower eccentricity. Finally, we find no evidence of other companions in the system (in the inner or the outer regions). Thus, the origin of the high-eccentricity of Gaia-6 B remains unclear.