The creation of multiple images by a gravitational wave
  We describe gravitational lensing by a gravitational wave, in the regime in which multiple images of a light source are created. We adapt the vector formalism employed for ordinary gravitational lenses to the case of a non-stationary spacetime, and we derive an approximate condition for multiple imaging. It is shown that certain astrophysical sources of gravitational waves satisfy this condition. 
Valerio Faraoni (University of Victoria)