Gravitational waves and the conformal transformation

We study the problem of the behaviour of cosmological gravitational waves u nder conformal transformations. In spite of the apparent triviality of this question, the informations we can obtain from gravitational waves in the s o-called Einstein's and Jordan's frame are not the same, mainly with respec t to the choice of the initial conditions and of graviton production. The o nly exception seems to occur in string cosmology due to the duality propert ies.