  We extend the rotationally invariant formulation of the slave-boson method to
superconducting states. This generalization, building on the recent work by
Lechermann et al. [Phys. Rev. B {\bf 76}, 155102 (2007)], allows to study
superconductivity in strongly correlated systems. We apply the formalism to a
specific case of strongly correlated superconductivity, as that found in a
multi-orbital Hubbard model for alkali-doped fullerides, where the
superconducting pairing has phonic origin, yet it has been shown to be favored
by strong correlation owing to the symmetry of the interaction. The method
allows to treat on the same footing the strong correlation effects and the
interorbital interactions driving superconductivity, and to capture the physics
of strongly correlated superconductivity, in which the proximity to a Mott
transition favors the superconducting phenomenon.
