  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.
