  This letter is a generalization of previous results on gravitational waves
(GWs) from f(R) theories of gravity. In some previous papers, particular f(R)
theories have been linearized for the first time in the literature. Now, the
process is further generalized, showing that every f(R) theory can be
linearized producting a third massive mode of gravitational radiation. In this
framework, previous results are particular cases of the more general problem
that is discussed in this letter. The potential detectability of such massive
GWs with LISA is also discussed with the auxilium of longitudinal response
functions.
