Example of a light curve and the CCF of the brightness variations in two photometric bands. Top: B band light curve of V603 Aql observed on 1983 July 18. The solid line is a fit of a high-order polynomial to the data. Centre: residuals between the original light curve and the polynomial fit. Bottom left: CCF of the light curve shown in the top panel with the simultaneously observed light curve in the U band (broken line) and of the residuals between the original data and the polynomial fit (solid line). Bottom right: enlarged section of the maximum of the CCF shown as a solid line in the lower left panel (dots) together with a fit of a high-order polynomial to the data (broken curve). The broken vertical line indicates the location of the maximum of the fitted polynomial.