Top: difference of the normalized spectra obtained in time intervals r₁ and c (squares). The solid line shows the sum of our best-fit sinusoidal plus linear model components. Vertical dashed (green) lines indicate the start and vertical dotted (red) lines indicate the end of an echelle order. Bottom: the residuals obtained after subtracting the model shown in the top panel from the data. The solid (red) line shows a smoothed synthetic difference spectrum.