Variability of observed Stokes q and u parameters for AO Cas in B, V, and R passbands shown in the top, middle, and bottom panels, respectively, which are phase-folded at the orbital period of 3.52 days. Fourier fit curves (see Sect. 3.3) are shown in solid lines and the best fit Fourier coefficients are given in Table 5. For the majority of data points, the length of ±σ error bars is shorter than the size of the plotting symbols.