  The nonlinear interaction between electromagnetic, electrostatic and
gravitational waves in a Vlasov plasma is reconsidered. By using a orthonormal
tetrad description the three-wave coupling coefficients are computed. Comparing
with previous results, it is found that the present theory leads to algebraic
expression that are much reduced, as compared to those computed using a
coordinate frame formalism. Furthermore, here we calculate the back-reaction on
the gravitational waves, and a simple energy conservation law is deduced in the
limit of a cold plasma.
