  In this paper we report the discovery of TOI-220 $b$, a new sub-Neptune
detected by the Transiting Exoplanet Survey Satellite (TESS) and confirmed by
radial velocity follow-up observations with the HARPS spectrograph. Based on
the combined analysis of TESS transit photometry and high precision radial
velocity measurements we estimate a planetary mass of 13.8 $\pm$ 1.0
M$_{Earth}$ and radius of 3.03 $\pm$ 0.15 R$_{Earth}$, implying a bulk density
of 2.73 $\pm$ 0.47 $\textrm{g cm}^{-3}$. TOI-220 $b$ orbits a relative bright
(V=10.4) and old (10.1$\pm$1.4 Gyr) K dwarf star with a period of $\sim$10.69
d. Thus, TOI-220 $b$ is a new warm sub-Neptune with very precise mass and
radius determinations. A Bayesian analysis of the TOI-220 $b$ internal
structure indicates that due to the strong irradiation it receives, the low
density of this planet could be explained with a steam atmosphere in
radiative-convective equilibrium and a supercritical water layer on top of a
differentiated interior made of a silicate mantle and a small iron core.

Earth and Planetary Astrophysics