Gravity from superstrings

We discuss the four-dimensional theory of gravity which arises from compactifications of the heterotic string. Its main feature is that the dilaton field is coupled to all non-gravitational matter in the Universe. Its cosmological solutions correspond to General Relativity or departures therefrom given by a JBD-like theory labelled by an ω-parameter. Unlike in ordinary JBD gravity (where the scalar field is decoupled from the non-gravitational sector), bounds on the time variation of the gauge coupling constants provide very strong constraints on ω. In particular data from the Oklo reactor translate into ω > 1015.