The string dilation and a least coupling principle

It is pointed out that string-loop modifications of the low-energy matter couplings of the dilation may provide a mechanism for fixing the vacuum expectation value of a massless dilation in a way which is naturally compatible with existing experimental data. Under a certain assumption of universality of the dilation coupling functions, the cosmological evolution of the graviton-dilaton-matter system is shown to drive the dilaton towards values where it decouples from matter ("Least Coupling Principle"). Quantitative estimates are given of the residual strength, at the present cosmological epoch, of the coupling to matter of the dilaton. The existence of a weakly coupled massless dilaton entails a large spectrum of small, but non-zero, observable deviations from general relativity. In particular, our results provide a new motivation for trying to improve by several orders of magnitude the various experimental tests of Einstein's Equivalence Principle (universality of free fall, constancy of the constants, etc.).