2006A&A...452...25U

The time delays between the components of a lensed quasar are basic tools for analysing the expansion of the Universe and the structure of the main lens galaxy halo. In this paper, we focus on the variability and time delay of the double system SBS 0909+532A,B as well as the time behaviour of the field stars. We use VR optical observations of SBS 0909+532A, B and the field stars in 2003. The frames were taken at Calar Alto, Maidanak, and Wise observatories, and the VR light curves of the field stars and quasar components were derived from aperture and point-spread function fitting methods. We measured the R-band time delay of the system from the χ² and dispersion techniques and 1000 synthetic light curves based on the observed records. One nearby field star (SBS 0909+532c) was found to be variable, and the other two nearby field stars are non-variable sources. With respect to the quasar components, the R-band records seem more reliable and are more densely populated than the V-band ones. The observed R-band fluctuations permit a pre-conditioned measurement of the time delay. From the χ² minimization, if we assume that the quasar emission is observed first in B and afterwards in A (in agreement with basic observations of the system and the corresponding predictions), we obtain Δτ_BA=-45^+ 1^_–11days (95% confidence interval). The dispersion technique leads to a similar delay range. A by-product of the analysis is the determination of a totally corrected flux ratio in the R band (corrected by the time delay and the contamination due to the galaxy light). Our 95% measurement Δm_BA=m_B(t+Δτ_BA)-m_A(t)=0.575±0.014mag is in excellent agreement with previous results from contaminated fluxes at the same time of observation.