  Gravitational waves in the presence of a non-minimal curvature-matter
coupling are analysed, both in the Newman-Penrose and perturbation theory
formalisms. Considering a cosmological constant as a source, the non-minimally
coupled matter-curvature model reduces to $f(R)$ theories. This is in good
agreement with the most recent data. Furthermore, a dark energy-like fluid is
briefly considered, where the propagation equation for the tensor modes differs
from the previous scenario, in that the scalar mode equation has an extra term,
which can be interpreted as the longitudinal mode being the result of the
mixture of two fundamental excitations $\delta R$ and $\delta \rho$.
