BV curves of XY Boo (see the list in Table 5). The phase is plotted according to our quadratic ephemeris (see Table 4). The areas of points are proportional to their weights. Open squares are normal points, each of which represents the mean of about 50 measurements. Grey lines are fits by our phenomenological model (see Eq. (18) and Table 3). ΔB and ΔV display residuals of BV magnitudes and normal points from the LC phenomenological model. The scale of residuals is two times larger than the measure of BV LCs.