O(2,2) transformations and the string Geroch group

The 1-loop string background equations with axion and dilaton fields are shown to be integrable in four dimensions in the presence of two commuting Killing symmetries and δc = 0. Then, in analogy with reduced gravity, there is an infinite group that acts on the space of solutions and generates non-trivial string backgrounds from flat space. The usual O(2,2) and S-duality transformations are just special cases of the string Geroch group, which is infinitesimally identified with the O(2,2) current algebra. We also find an additional Z2 symmetry interchanging the field content of the dimensionally reduced string equations. The method for constructing multi-soliton solutions on a given string background is briefly discussed. ((orig.))