GLITP optical monitoring of QSO 0957+561: VR light curves and variability. - Ullán, A.; Goicoechea, L. J.; Mun˜oz, J. A.; Mediavilla, E.; Serra-Ricart, M.; Puga, E.; Alcalde, D.; Oscoz, A.; Barrena, R. (Monthly Notices of the Royal Astronomical Society, 2003)