Abstract:
We show that T-duality can be broken by nonperturbative
effects in string coupling. The T-duality in question is
that of the 2-torus when the heterotic string is
compactified on K3 x T2. This case is compared carefully to
a situation where T-duality appears to work. A holonomy
argument is presented to show that T-dualities (and general
U-dualities) should only be expected for large amounts of
supersymmetry. This breaking of R <-> 1/R symmetry raises
some interesting questions in string theory which we
discuss. Finally we discuss how the classical modular group
of a 2-torus appears to be broken too.