  We report the discovery and characterisation of the transiting mini-Neptune
HD~207496~b (TOI-1099) as part of a large programme that aims to characterise
naked core planets. We obtained HARPS spectroscopic observations, one
ground-based transit, and high-resolution imaging which we combined with the
TESS photometry to confirm and characterise the TESS candidate and its host
star. The host star is an active early K dwarf with a mass of $0.80 \pm
0.04\,$M$_\odot$, a radius of $0.769 \pm 0.026\,$R$_\odot$, and a G magnitude
of 8. We found that the host star is young, $\sim 0.52\,$ Myr, allowing us to
gain insight into planetary evolution. We derived a planetary mass of $6.1 \pm
1.6\,\mathrm{M}_E$,\, a planetary radius of $2.25 \pm 0.12\,\mathrm{R}_E$,\ and
a planetary density of $\rho_p = 3.27_{-0.91}^{+0.97}\,\mathrm{g.cm^{-3}}$.
From internal structure modelling of the planet, we conclude that the planet
has either a water-rich envelope, a gas-rich envelope, or a mixture of both. We
have performed evaporation modelling of the planet. If we assume the planet has
a gas-rich envelope, we find that the planet has lost a significant fraction of
its envelope and its radius has shrunk. Furthermore, we estimate it will lose
all its remaining gaseous envelope in $\sim 0.52\,$ Gyr. Otherwise, the planet
could have already lost all its primordial gas and is now a bare ocean planet.
Further observations of its possible atmosphere and/or mass-loss rate would
allow us to distinguish between these two hypotheses. Such observations would
determine if the planet remains above the radius gap or if it will shrink and
be below the gap.
