Dark matter, time-varyingG, and a dilaton field

We demonstrate the observational viability of having an exactly massless dilaton field couple with gravitational strength to most matter in the Universe. This is done by constructing a generalized Jordan-Brans-Dicke model in which the scalar couples with different strengths to visible and to conjectured ``dark'' matter. In this model, improved Ġ \. G measurements may provide nontrivial bounds on the coupling constants of the dilaton to matter.