  Highly irradiated planets in the hot Neptune desert are usually either small
(R < 2 Rearth) and rocky or they are gas giants with radii of >1 Rjup. Here, we
report on the intermediate-sized planet TOI-2196 on a 1.2 day orbit around a
G-type star discovered by TESS in sector 27. We collected 42 radial velocity
measurements with the HARPS spectrograph to determine the mass. The radius of
TOI-2196 b is 3.51 +/- 0.15 Rearth, which, combined with the mass of 26.0 +/-
1.3 Mearth, results in a bulk density of 3.31+0.51-0.43 g/cm3. Hence, the
radius implies that this planet is a sub-Neptune, although the density is twice
than that of Neptune. A significant trend in the HARPS radial velocities points
to the presence of a distant companion with a lower limit on the period and
mass of 220 days and 0.65 Mjup, respectively, assuming zero eccentricity. The
short period of planet b implies a high equilibrium temperature of 1860 +/- 20
K, for zero albedo and isotropic emission. This places the planet in the hot
Neptune desert, joining a group of very few planets in this parameter space
discovered in recent years. These planets suggest that the hot Neptune desert
may be divided in two parts for planets with equilibrium temperatures of > 1800
K: a hot sub-Neptune desert devoid of planets with radii of 1.8-3 Rearth and a
sub-Jovian desert for radii of 5-12 Rearth. More planets in this parameter
space are needed to further investigate this finding. Planetary interior
structure models of TOI-2196 b are consistent with a H/He atmosphere mass
fraction between 0.4 % and 3 %, with a mean value of 0.7 % on top of a rocky
interior. We estimated the amount of mass this planet might have lost at a
young age, and we find that while the mass loss could have been significant,
the planet had not changed in terms of character: it was born as a small
volatile-rich planet, and it remains one at present.
