Top: small dots represent between all possible permutations of measurements i,j with τ = | t_j − t_i |. Larger dots mark the averages inside a bin of one day with their uncertainty plotted as a continuous black line. The red straight line at 44.4° corresponds to the linear fit of all data Γ_BV = 0.979. Bottom: the structure function S of the variability; black boxes for B-band and red diamonds for V-band.