SYMMETRIES OF 4-DIMENSIONAL STRING EFFECTIVE ACTION WITH COSMOLOGICALCONSTANT

Classical solutions for a four-dimensional Minkowskian string effectiv e action and an Euclidean one with cosmological constant term are deri ved. The former corresponds to electrovac solutions whereas the later solutions are identified as gravitational instanton solutions for Fubi ni-Study metric. The symmetries of the effective actions are identifie d and new classical solutions are generated by implementing appropriat e noncompact transformations. The S-duality transformations on the equ ations of motion are discussed and it is found that they are S-duality noninvariant due to the presence of cosmological constant term.