GRAVITATIONAL LENS TIME DELAYS AND GRAVITATIONAL-WAVES

Using Fermat's principle, we analyze the effects of very long waveleng th gravitational waves upon the images of a gravitationally lensed qua sar. We show that the lens equation in the presence of gravity waves i s equivalent to that of a lens with a different alignment between sour ce, deflector, and observer in the absence of gravity waves. Contrary to a recent claim, we conclude that measurements of time delays in gra vitational lenses cannot serve as a method to detect or constrain a st ochastic background of gravitational waves of cosmological wavelengths , because the wave-induced time delay is observationally indistinguish able from an intrinsic time delay due to the lens geometry.