Nonperturbative SL(2,Z) (p,q)-strings manifestly realized on the quantum   M2
  The SL(2,Z) duality symmetry of IIB superstring is naturally realized on the D=11 supermembrane restricted to have central charges arising from a nontrivial wrapping. This supermembrane is minimally immersed on the target space (MIM2). The hamiltonian of the MIM2 has a discrete quantum spectrum. It is manifestly invariant under the SL(2,Z) symmetry associated to the conformal symmetry on the base manifold and under a SL(2,Z) symmetry on the moduli of the target space. The mass contribution of the string states on the MIM2 is obtained by freezing the remaining degrees of freedom. It exactly agrees with the perturbative spectrum of the (p,q) IIB and IIA superstring compactified on a circle. We also construct a MIM2 in terms of a dual target space, then a (p,q) set of non-perturbative states associated to the IIA superstring is obtained. 
M.P. Garcia del Moral, I. Martin, A. Restuccia