  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 Z2 reflection symmetry which leads to a
further SL(2,R) sub-group. The symmetries are employed to analyse a general
class of spatially homogeneous cosmological solutions with non-trivial
Ramond-Ramond fields.
