  Scalar field models with non-standard kinetic terms have been proposed in the
context of k-inflation, of Born-Infeld lagrangians, of phantom energy and, more
in general, of low-energy string theory. In general, scalar fields are expected
to couple to matter inducing a new interaction. In this paper I derive the
cosmological perturbation equations and the Yukawa correction to gravity for
such general models. I find three interesting results: first, when the field
behaves as phantom energy (equation of state less than -1) then the coupling
strength is negative, inducing a long-range repulsive force; second, the dark
energy field might cluster on astrophysical scales; third, applying the
formalism to a Brans-Dicke theory with general kinetic term it is shown that
its Newtonian effects depend on a single parameter that generalizes the
Brans-Dicke constant.
