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We present the results of a photometric CCD monitoring of the gravitationallens system UM 673, that took place from 1995 to 2000. In total, thedoubly-imaged quasar was observed in the R-band during 29 photometric nights,using optical telescopes with dimensions in the range 0.6 m to 1.3 m. Wedetected a significant variability in the total light of the UM 673 system,that is, in the light of the two QSO images plus the lensing galaxy. Withrespect to the magnitude of the gravitational lens system at its discovery, in1986, UM 673 was 0.3 magnitude brighter. Furthermore, our December 1996measurements show that between November 1995 and October 1997 the system becameeven brighter, reaching a magnitude difference of 0.5 magnitude with respect toits discovery value. We also present R magnitudes and V-R colours of sevenfield stars situated in the vicinity of the lens, based on a 3.5-monthmonitoring during the year 2000.Comment: 15 pages, 2 figures, accepted for publication in Astronomical Journa

Photometric Variability of the Gravitational Lens 0142−100 (UM 673)