  The recently discovered $O(d,d)$ symmetry of the space of slowly varying
cosmological string vacua in $d+1$ dimensions is shown to be preserved in the
presence of bulk string matter. The existence of $O(d,d)$ conserved currents
allows all the equations of string cosmology to be reduced to first-order
differential equations. The perfect-fluid approximation is not
$O(d,d)$-invariant, implying that stringy fluids possess in general a
non-vanishing viscosity.
