  We present the results of a photometric CCD monitoring of the gravitational
lens system UM 673, that took place from 1995 to 2000. In total, the
doubly-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. We
detected 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. With
respect to the magnitude of the gravitational lens system at its discovery, in
1986, UM 673 was 0.3 magnitude brighter. Furthermore, our December 1996
measurements show that between November 1995 and October 1997 the system became
even brighter, reaching a magnitude difference of 0.5 magnitude with respect to
its discovery value. We also present R magnitudes and V-R colours of seven
field stars situated in the vicinity of the lens, based on a 3.5-month
monitoring during the year 2000.
