  We present ab initio many-body calculations of the optical absorption in bulk
graphite, graphene and bilayer of graphene. Electron-hole interaction is
included solving the Bethe-Salpeter equation on top of a GW quasiparticle
electronic structure. For all three systems, we observe strong excitonic
effects at high energy, well beyond the continuum of \pi -> \pi * transitions.
In graphite, these affect the onset of \sigma -> \sigma * transitions. In
graphene, we predict an excitonic resonance at 8.3 eV arising from a background
continuum of dipole forbidden transitions. In the graphene bilayer, the
resonance is shifted to 9.6 eV. Our results for graphite are in good agreement
with experiments.
