Wannier excitons signalling strong Coulomb coupling in graphene J.H. Grönqvist, T. Stroucken, M. Lindberg and S.W. Koch Eur. Phys. J. B, 85 12 (2012) 395 Published online: 03 December 2012 DOI: 10.1140/epjb/e2012-30593-0