Low-frequency pulse profile variation in PSR B2217+47: evidence for echoes from the interstellar medium. - Michilli, D.; Hessels, J. W. T.; Donner, J. Y.; Grießmeier, J.-M.; Serylak, M.; Shaw, B.; Stappers, B. W.; Verbiest, J. P. W.; Deller, A. T.; Driessen, L. N.; Stinebring, D. R.; Bondonneau, L.; et al. (Monthly Notices of the Royal Astronomical Society, 2018)