  Former extensive studies of superconductivity in the \textit{A}$_{3}$C$_{60}$
compounds, where \textit{A} is an alkali, have led to consider that Bardeen
Cooper Schrieffer (BCS) electron-phonon pairing prevails in those compounds,
though the incidence of electronic Coulomb repulsion has been highly debated.
The discovery of two isomeric fulleride compounds Cs$_{3}$C$_{60}$ which
exhibit a transition with pressure from a Mott insulator (MI) to a
superconducting (SC) state clearly re-opens that question. Using pressure ($p$)
as a single control parameter of the C$_{60}$ balls lattice spacing, one can
now study the progressive evolution of the SC properties when the electronic
correlations are increased towards the critical pressure $p_{c}$ of the Mott
transition. We have used $^{13}$C and $^{133}$Cs NMR measurements on the cubic
phase A15-Cs$_{3}$C$_{60}$ just above $p_{c}=5.0(3)$ kbar, where the SC
transition temperature $T_{c}$ displays a dome shape with decreasing cell
volume. From the $T$ dependence below $T_{c}$ of the nuclear spin lattice
relaxation rate $(T_{1})^{-1}$ we determine the electronic excitations in the
SC state, that is $2\Delta$, the SC gap value. We find that $2\Delta $
increases with decreasing $p$ towards $p_{c}$, where $T_{c}$ decreases on the
SC dome, so that $2\Delta /k_{B}T_{c}$ increases regularly upon approaching the
Mott transition. These results bring clear evidence that the increasing
correlations near the Mott transition are not significantly detrimental to SC.
They rather suggest that repulsive electron interactions might even reinforce
elecron-phonon SC, being then partly responsible for the large $T_{c}$ values,
as proposed by theoretical models taking the electronic correlations as a key
ingredient.
