SIMBAD references

We present observations of wavelength-dependent flux ratios for four gravitational lens systems (SDSS J1650+4251, HE 0435-1223, FBQ 0951+2635, and Q 0142-100) obtained with the Nordic Optical Telescope. The use of narrowband photometry, as well as the excellent seeing conditions during the observations, allows us to study their chromatic behavior. For SDSS J1650+4251, we determine the extinction curve of the dust in the z_L= 0.58 lens galaxy and find that the 2175 Å feature is absent. In the case of HE 0435-1223, we clearly detect chromatic microlensing. This allows us to estimate the wavelength-dependent size of the accretion disk. We find an R-band disk size of r^R_s= 13 ±5 light days for a linear prior on r^R_s and r^R_s= 7±6 light days for a logarithmic prior. For a power-law size-wavelength scaling of r_s∝ λ^ p ^, we were able to constrain the value of the exponent to p = 1.3±0.3 for both r^R_spriors, which is in agreement with the temperature profiles of simple thin disk models (p = 4/3).