Variability in the observed Stokes parameters of Cyg X-1, obtained during season 2 in B, V, and R bands (top, middle, and bottom panels, respectively). Each circle corresponds to the average value, calculated within a 30-min bin. The 1σ errors are smaller than the symbol size. The solid black lines correspond to the best fit with the Fourier series given by Eq. (2). The dashed red lines in the middle panels show the best fit of PMO historical V -band data with the same model, shifted vertically to overlap our data.