  Detecting planetary signatures in radial velocity time-series of young stars
is challenging due to their inherently strong stellar activity. However, it is
possible to learn information about the properties of the stellar signal by
using activity indicators measured from the same stellar spectra used to
extract radial velocities. In this manuscript, we present a reanalysis of
spectroscopic HARPS data of the young star K2-233, which hosts three transiting
planets. We perform a multidimensional Gaussian Process regression on the
radial velocity and the activity indicators to characterise the planetary
Doppler signals. We demonstrate, for the first time on a real dataset, that the
use of a multidimensional Gaussian Process can boost the precision with which
we measure the planetary signals compared to a one-dimensional Gaussian Process
applied to the radial velocities alone. We measure the semi-amplitudes of
K2-233 b, c, and d as 1.31(-0.74)(+0.81), 1.81(-0.67)(+0.71), and
2.72(-0.70)(+0.66) m/s, which translates into planetary masses of
2.4(-1.3)(+1.5), 4.6(-1.7)(+1.8), and 10.3(-2.6)(+2.4), respectively. These new
mass measurements make K2-233 d a valuable target for transmission spectroscopy
observations with JWST. K2-233 is the only young system with two detected inner
planets below the radius valley and a third outer planet above it. This makes
it an excellent target to perform comparative studies, to inform our theories
of planet evolution, formation, migration, and atmospheric evolution.
