  GJ 9827 is a bright, nearby K7V star orbited by two super-Earths and one
mini-Neptune on close-in orbits. The system was first discovered using K2 data
and then further characterized by other spectroscopic and photometric
instruments. Previous literature studies provide several mass measurements for
the three planets, however, with large variations and uncertainties. To better
constrain the planetary masses, we added high-precision radial velocity
measurements from ESPRESSO to published datasets from HARPS, HARPS-N, and HIRES
and we performed a Gaussian process analysis combining radial velocity and
photometric datasets from K2 and TESS. This method allowed us to model the
stellar activity signal and derive precise planetary parameters. We determined
planetary masses of $M_b = 4.28_{-0.33}^{+0.35}$ M${_\oplus}$, $M_c =
1.86_{-0.39}^{+0.37}$ M${_\oplus}$, and $M_d = 3.02_{-0.57}^{+0.58}$
M${_\oplus}$, and orbital periods of $1.208974 \pm 0.000001$ days for planet b,
$3.648103_{-0.000010}^{+0.000013}$ days for planet c, and $6.201812 \pm
0.000009$ days for planet d. We compared our results to literature values and
found that our derived uncertainties for the planetary mass, period, and radial
velocity amplitude are smaller than the previously determined uncertainties. We
modeled the interior composition of the three planets using the
machine-learning-based tool ExoMDN and conclude that GJ 9827 b and c have an
Earth-like composition, whereas GJ 9827 d has an hydrogen envelope, which,
together with its density, places it in the mini-Neptune regime.
