Variability of intrinsic Stokes parameters q_int (left column) and u_int (right column), in B, V, and R bands (top, middle and bottom panels, respectively) with the orbital phase folded with the period P_orb = 26.496 d. The filled squares with 1σ errors correspond to the average values of the individual observations (gray circles with error bars) and the standard errors of the mean calculated within phase bins of width Δϕ = 0.091. The vertical dashed lines correspond to the phases of periastron (ϕ = 0.275) and apastron (ϕ = 0.775) as derived by Aragona et al. (2009). The red dashed lines show the best fit with Fourier series given by Eq. (1). The black solid and blue dotted lines correspond to the best fit model of a scattering cloud on an eccentric orbit from Sect. 3.3, for an Ω constrained close to θ_int and with a free Ω, respectively.