  At low energy, the four-dimensional effective action of the ekpyrotic model
of the universe is equivalent to a slightly modified version of the pre big
bang model. We discuss cosmological perturbations in these models. In
particular we address the issue of matching the perturbations from a collapsing
to an expanding phase in full generality. We show that, generically, one
obtains $n=0$ for the spectrum of scalar perturbations in the original pre big
model (with vanishing potential). When an exponential potential for the dilaton
is included, a scale invariant spectrum ($n=1$) of adiabatic scalar
perturbations is produced under very generic matching conditions, both in a
modified pre big bang and ekpyrotic scenario. We also derive general results
valid for power law scale factors matched to a radiation dominated era.
