  We propose to uncover the signature of a stringy era in the primordial
Universe by searching for a prominent peak in the relic graviton spectrum. This
feature, which in our specific model terminates an $\omega^3$ increase and
initiates an $\omega^{-7}$ decrease, is induced during the so far overlooked
bounce of the scale factor between the collapsing deflationary era (or pre-Big
Bang) and the expanding inflationary era (or post-Big Bang). We evaluate both
analytically and numerically the frequency and the intensity of the peak and we
show that they may likely fall in the realm of the new generation of
interferometric detectors. The existence of a peak is at variance with
ordinarily monotonic (either increasing or decreasing) graviton spectra of
canonical cosmologies; its detection would therefore offer strong support to
string cosmology.
