Abstract. We present a catalog of secondary photometric standard stars in the neighborhood of 14 gravitationally lensed quasars. These stars were verified to be non variable using long-term monitoring. The instrumental magnitudes of the new standard stars have been transformed to the Johnson-Cousins BV(RI) c photometric system. For ten gravitational lenses (GLs) we also provide the BV(RI) c mean magnitudes of the integrated flux of all the lens components, for the epochs of the photometric calibration.

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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 doublyimaged 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 2000.1 Based on observations made with a) the 60 cm Bochum and the ESO 90 cm Dutch telescopes, at La

Abstract. Differences ∆V of magnitude and ∆(B−V ) and ∆(U −B) colours, as well as separations and position angles of 42 relatively close double stars are presented. The selection criteria of our sample are: the relatively small separation (between 2.5 and 5 arcseconds), magnitude difference under 1.25 mag, and mainly G-type primaries. The CCD observations, performed at the 90 cm Dutch telescope at ESO, were made using the Bessel U , B and V filters; the astrometry was done in the V filter only.From the analysis of the photometric data we conclude that 16 binaries in our sample have components with practically the same T eff , since they have almost the same colours; one binary has components with identical characteristics. We also note the good internal accuracy of the astrometric CCD measurements.

The study of double stars has since long been recognized as a basic key to the understanding of star formation and stellar evolution. Moreover, close visual double stars have always been systematically neglected in photometric observational programmes although they contain an important part of physically associated systems. It is then timely to organize major observational programmes of these objects for a number of good reasons:1.The frequency of double stars is continuously reviewed and the rate of their detection is steadily increasing — both from ground–based and space observations — in such a way that they no longer can be discarded in any models of galactic structure;2.Space observations (HIPPARCOS, HST) significantly improve the quality and the importance of stellar samples and permit to better take into account some of the selection effects;3.The high–quality astrometric (and also photometric) data that will be provided for such systems by the space observations should be matched with accurate and homogeneous complementary astrophysical information such as colour indices and spectral classification. Such information for close visual double stars is unfortunately almost non-existent but are now being more easily accessible with the use of CCD detectors.