Symmetries for generating string cosmologies

We discuss the symmetry properties of the low-energy effective action of the type IIB superstring that may be employed to derive four-dimensional solutions. A truncated effective action, compactified on a six-torus, but including both Neveu-Schwarz-Neveu-Schwarz and Ramond-Ramond field strengths, can be expressed as a non-linear sigma model which is invariant under global SL(3,R) transformations. This group contains as a sub-group the SL(2,R) symmetry of the ten-dimensional theory and a discrete Z(2) reflection symmetry which leads to a further SL(2,R) sub-group. The symmetries are employed to analyze a general class of spatially homogeneous cosmological solutions with non-trivial Ramond-Ramond fields.