K3 surfaces, modular forms, and non-geometric heterotic   compactifications
  We construct non-geometric compactifications by using the F-theory dual of the heterotic string compactified on a two-torus, together with a close connection between Siegel modular forms of genus two and the equations of certain K3 surfaces. The modular group mixes together the K\"ahler, complex structure, and Wilson line moduli of the torus yielding weakly coupled heterotic string compactifications which have no large radius interpretation. 
Andreas Malmendier and David R. Morrison
hep-th math.AG