Gravitational waves in the presence of a non-minimal curvature-mattercoupling are analysed, both in the Newman-Penrose and perturbation theoryformalisms. Considering a cosmological constant as a source, the non-minimallycoupled matter-curvature model reduces to $f(R)$ theories. This is in goodagreement with the most recent data. Furthermore, a dark energy-like fluid isbriefly considered, where the propagation equation for the tensor modes differsfrom 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 themixture of two fundamental excitations $\delta R$ and $\delta \rho$.
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