The optical variability of QSOs. II. The wavelength dependence

The long-term variability of a sample of 149 optically selected QSOs in the field of the Selected Area 94 has been studied in the R-band. The relations between variability and luminosity and between variability and redshift have been investigated by means of ``robust'' statistical estimators, allowing to disentangle the effects of the measurement errors. The results are compared with the corresponding properties of the variability in the B-band for the same sample. An anti-correlation between the R-band variability and the intrinsic luminosity is found, analogously to what is observed in the B-band. The amplitude of the R-band variability turns out to be smaller (of a factor $1.13 \pm 0.05$) than the B-band variability. The implications in terms of the black-hole, starburst and microlensing models are discussed.