Structure of dualities in bosonic string theory

The nature of duality symmetries is explored in closed bosonic string theory, particularly in the case of a four-dimensional target space admitting a one-parameter isometry. It appears that the S-duality of string theory behaves analogously to the Ehlers' symmetry of General Relativity. Furthermore, it is demonstrated that the O(1,1) target space duality arising from the isometry interchanges the roles of these two symmetries. The inclusion of the tachyon field is shown to be consistent with T-duality but incompatible with S-duality. Finally, extrapolating to dimensions other than four, the effective action is found to be invariant under a larger group of symmetries than the usual O(1,1).