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At low energy, the four-dimensional effective action of the ekpyrotic modelof the universe is equivalent to a slightly modified version of the pre bigbang model. We discuss cosmological perturbations in these models. Inparticular we address the issue of matching the perturbations from a collapsingto an expanding phase in full generality. We show that, generically, oneobtains $n=0$ for the spectrum of scalar perturbations in the original pre bigmodel (with vanishing potential). When an exponential potential for the dilatonis included, a scale invariant spectrum ($n=1$) of adiabatic scalarperturbations is produced under very generic matching conditions, both in amodified pre big bang and ekpyrotic scenario. We also derive general resultsvalid for power law scale factors matched to a radiation dominated era.Comment: 11 pages, 1 figure, revised version with small corrections to match  version in print. Results and conclusions unchange

Adiabatic perturbations in pre-big bang models: Matching conditions and scale invariance