Hulse and Taylor's observation of PSR B1913+16 provides indirect evidence for the existence of gravitational waves ("GW") in accordance with GR. Simulations show recoil of merging black holes due to the momentum of the GW they emit during the inspiral. http://www.black-holes.org/explore2.html GW propagate at c as disturbances on a flat background and as gravitons are massless, the usual rule E=p should apply. Other than being quadrupole radiation and gravitons being spin 2 accordingly, the picture is analogous to EM waves and photons. Consider two binary star systems, A-B and C-D with an observer O somewhere between: A-B O C-D In the special case of four equal mass stars orbiting in a common plane with equal periods, GW of equal amplitude will travel in both directions from the two source systems resulting in a standing wave pattern at the location of O. More generally, if the periods differ, O can select a motion along the line between the systems which equalises the observed periods via the Doppler effect and if the masses differ, he can select a position closer to the lighter pair to equalise the amplitudes. We can calculate the effective mass of an aggregate formed from a pair of photons of equal frequency moving anti-parallel. Because the energies add but the momenta cancel so the resulting mass found from m2=E2-p2 is non-zero, i.e. while one photon has no mass, a pair does. It seems to me that the same applies to the GW scenario, i.e. the standing wave component at location O formed from waves of equal amplitude and frequency moving antiparallel has finite energy but no net momentum and thus should have an effective mass. Is this correct and if so would that mass also be a source of gravitation, or to put it another way, should the energy density of those waves be included in the stress-energy tensor?