Upper panels: equivalent widths W_BAL, W_NALb, and W_NALr of the C IV absorptions as a function of time. Bottom panel: R band light curve ΔR_QS of APM 08279+5255 normalised to the reference star S adopted in the Asiago monitoring campaign (upper curve). For comparison, the magnitude difference ΔR_1S between the star S₁ and our reference star S is also reported as an indication of the photometric accuracy. Photometry, relative to the star S, is computed from magnitude differences in our data, while it is recalculated according to Eq. (1) for Lewis et al. (1999) data.