E. van Dessel

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

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 (QSO) was observed in the R band during 29 photometric nights, using optical telescopes with dimensions in the range 0.6–1.3 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 mag brighter. Furthermore, our 1996 December measurements show that between 1995 November and 1997 October the system became even brighter, reaching a magnitude difference of 0.5 mag with respect to its discovery value. We also present R magnitudes and V-R colors of seven field stars situated in the vicinity of the lens, based on a 3.5 month monitoring during the year 2000.

Photometric Monitoring of the Lensed QSO 0957+561

We present our first results of a photometric monitoring project of the twin quasar 0957+561. This project aims mainly at the improvement of the determination of the time delay ΔT(A,B) for this gravitational lens, since the “time delay controversy on QSO 0957+561 (is) not yet decided” (Pelt et al. 1994). In addition, the quite large field of the CCD used allows also a long-term astrometric and photometric study of stars and galaxies in the field within a radius of about 10 arcminutes around the lens.