  Scalar fields with inverse power-law effective potentials may provide a
negative pressure component to the energy density of the universe today, as
required by cosmological observations. In order to be cosmologically relevant
today, the scalar field should have a mass
  $m_\phi = O(10^{-33} {\mathrm eV})$, thus potentially inducing sizable
violations of the equivalence principle and space-time variations of the
coupling constants. Scalar-tensor theories of gravity provide a framework for
accommodating phenomenologically acceptable ultra-light scalar fields. We
discuss non-minimally coupled scalar-tensor theories in which the scalar-matter
coupling is a dynamical quantity. Two attractor mechanisms are operative at the
same time: one towards the tracker solution, which accounts for the accelerated
expansion of the Universe, and one towards general relativity, which makes the
ultra-light scalar field phenomenologically safe today. As in usual
tracker-field models, the late-time behavior is largely independent on the
initial conditions. Strong distortions in the cosmic microwave background
anisotropy spectra as well as in the matter power spectrum are expected.
