Aspinwall, PS; Plesser, MR, *T-duality can fail*,
Journal of High Energy Physics, vol. 3 no. 8
(1999),
pp. XI-18 [hep-th/9905036] .
**Abstract:**

*We show that T-duality can be broken by non-perturbative effects in string coupling. The T-duality in question is that of the 2-torus when the heterotic string is compactified on K3xT2. 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.*