  The evolution of the rotational inhomogeneities is investigated in the
specific framework of four-dimensional pre-big bang models. While minimal
(dilaton-driven) scenarios do not lead to rotational fluctuations, in the case
of non-minimal (string-driven) models, fluid sources are present in the pre-big
bang phase. The rotational modes of the geometry, coupled to the divergenceless
part of the velocity field, can then be amplified depending upon the value of
the barotropic index of the perfect fluids. In the light of a possible
production of rotational inhomogeneities, solutions describing the coupled
evolution of the dilaton field and of the fluid sources are scrutinized in both
the string and Einstein frames. In semi-realistic scenarios, where the
curvature divergences are regularized by means of a non-local dilaton
potential, the rotational inhomogeneities are amplified during the pre-big bang
phase but they decay later on. Similar analyses can also be performed when a
contraction occurs directly in the string frame metric.
