In this paper, we investigate the gravitational waves in a particular class of alternative theories of gravity which is known as higher-derivative f (R,R, T ) gravity theory. In low curvature regime and by using Newman–Penrose analysis for the linear order of the field equations, we show that in addition to the usual plus and cross polarization modes which appear in general relativity, all other modes can exist too. Actually, according to our analysis, in such a theory by choosing the proper values from the parameter space of the model, a complete set of gravitational wave polarization modes can be produced in this concept.
