The graceful exit in pre-Big Bang string cosmology

We re-examine the graceful exit problem in the pre-Big Bang scenario of str ing cosmology, by considering the most general time-dependent classical cor rection to the lagrangean with up to four derivatives. By including possibl e forms for quantum loop corrections we examine the allowed region of param eter space for the coupling constants which enable our solutions to link sm oothly the two asymptotic low-energy branches of the pre-Big Bang scenario, and observe that these solutions can satisfy recently proposed entropic bo unds on viable singularity free cosmologies.