On the Moduli Space of the $T^6/Z_3$ Orbifold and Its Modular Group
  We describe the duality group $\Gamma=SU(3,3,Z)$ for the Narain lattice of the $T^6/Z_3$ orbifold and its action on the corresponding moduli space. A symplectic embedding of the momenta and winding numbers allows us to connect the orbifold lattice to the special geometry of the moduli space. As an application, a formal expression for an automorphic function, which is a candidate for a non--perturbative superpotential, is given. 
S. Ferrara, P. Fr\`e, P. Soriani