  The combination of the local-density approximation (LDA) with the
rotationally invariant slave-boson theory (RISB) is used to investigate the
realistic correlated electronic structure of Sr$_3$Ru$_2$O$_7$. From
Wannier-downfolding the low-energy band structure to a three-band model for the
Ru($t_{2g}$) states, the interacting problem is solved including intra- and
inter-orbital Hubbard terms as well as spin-flip and pair-hopping interactions.
Therewith it is possible to obtain valuable insight into the orbital
occupations, relevant local spin multiplets and the fermiology with increasing
correlation strength. Besides generic correlation-induced band-narrowing and
-shifting, an intriguing quasiparticle structure close to the Fermi level is
found in the neigborhood of the notorious $\gamma_2$ pocket in the Brillouin
zone. Along the $\Gamma$$-$$X$ direction in {\bf k}-space, that structure
appears very sensitive to electronic self-energy effects. The subtle
sensitivity, connected also its manifest multi-orbital character, may put this
very low-energy structure in context with the puzzling metamagnetic properties
of the compound.
