  We report on the detailed characterization of the HD 77946 planetary system.
HD 77946 is an F5 ($M_*$ = 1.17 M$_{\odot}$, $R_*$ = 1.31 R$_{\odot}$) star,
which hosts a transiting planet recently discovered by NASA's Transiting
Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS
photometry, high-resolution spectroscopic data from HARPS-N, and photometry
from CHEOPS, we measure the radius and mass from the transit and RV
observations, and find that the planet, HD 77946 b, orbits with period $P_{\rm
b}$ = $6.527282_{-0.000020}^{+0.000015}$ d, has a mass of $M_{\rm b} =
8.38\pm{1.32}$M$_\oplus$, and a radius of $R_{\rm b} =
2.705_{-0.081}^{+0.086}$R$_\oplus$. From the combination of mass and radius
measurements, and the stellar chemical composition, the planet properties
suggest that HD 77946 b is a sub-Neptune with a $\sim$1\% H/He atmosphere.
However, a degeneracy still exists between water-world and
silicate/iron-hydrogen models, and even though interior structure modelling of
this planet favours a sub-Neptune with a H/He layer that makes up a significant
fraction of its radius, a water-world composition cannot be ruled out, as with
$T_{\rm eq} = 1248^{+40}_{-38}~$K, water may be in a supercritical state. The
characterisation of HD 77946 b, adding to the small sample of
well-characterised sub-Neptunes, is an important step forwards on our journey
to understanding planetary formation and evolution pathways. Furthermore, HD
77946 b has one of the highest transmission spectroscopic metrics for small
planets orbiting hot stars, thus transmission spectroscopy of this key planet
could prove vital for constraining the compositional confusion that currently
surrounds small exoplanets.
