ON ROLLING MODULI SOLUTIONS IN STRING COSMOLOGY

Given a static string solution with some free constant parameters (''m oduli'') it may be possible to construct a time-dependent solution by just replacing the moduli by some functions of time. We present severa l examples when such a ''rolling moduli'' ansatz is consistent. In par ticular, the anisotropic D=4 cosmological solution of Nappi and Witten can be reinterpreted as a time-dependent generalisation of the (analy tic continuation of the) D=3 ''charged black string'' background with the ''charge'' changing with time. We find some new D=4 cosmological s olutions which are ''two rolling moduli'' generalisations of the previ ously known ones. We also comment on the interplay between duality tra nsformations and the replacement of moduli by functions of time.