Effective Theory of Interacting Dark Energy
  We present a unifying treatment of dark energy and modified gravity that
allows distinct conformal-disformal couplings of matter species to the
gravitational sector. In this very general approach, we derive the conditions
to avoid ghost and gradient instabilities. We compute the equations of motion
for background quantities and linear perturbations. We illustrate our formalism
with two simple scenarios, where either cold dark matter or a relativistic
fluid is nonminimally coupled. This extends previous studies of coupled dark
energy to a much broader spectrum of gravitational theories.
