The (so-called) duality symmetry of closed strings with d compactified dimensions is reformulated in geometrical terms as an invariance under a particular Z2 transformation acting on (constant) metric, torsion and gauge background fields. Fixed points of this transformation - moded by SLd(Z) - yield enlarged symmetries based on simply-laced gauge groups. Exact duality requires revision of naive compactification prescriptions in the effective low-energy action. It could also prove important for dynamical compactification in string theories.
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* Supported in part b', the Israeli Academy of Science anti Science Foundation