Test of the Equivalence Principle in the Dark Sector on Galactic Scales
  The Einstein Equivalence Principle is a fundamental principle of the theory
of General Relativity. While this principle has been thoroughly tested with
standard matter, the question of its validity in the Dark sector remains open.
In this paper, we consider a general tensor-scalar theory that allows to test
the equivalence principle in the Dark sector by introducing two different
conformal couplings to standard matter and to Dark matter. We constrain these
couplings by considering galactic observations of strong lensing and of
velocity dispersion. Our analysis shows that, in the case of a violation of the
Einstein Equivalence Principle, data favour violations through coupling
strengths that are of opposite signs for ordinary and Dark matter. At the same
time, our analysis does not show any significant deviations from General
Relativity.
