@article{GoicoecheaGilMerinoSerraRicartetal.2001,
  author    = {Luis Julian Goicoechea and Rodrigo Gil-Merino and M. Serra-Ricart and Alejandro Oscoz and D. Alcalde and E. Mediavilla},
  title     = {IAC gravitational lenses monitoring program : difference signals from 2.5 years of QSO 0957+561 observations},
  year      = {2001},
}