Cross correlation of B and Hα light curves for campaign 2011. The dotted lines indicate the error range (± 1σ) around the cross correlation. The centroid was calculated above the correlation level at r ≥ 0.8r_max. The histogram shows the distribution of the centroid lag obtained by cross correlating 2000 flux randomized and randomly selected subset light curves (FR/RSS method). The black shaded area marks the 68% confidence range used to calculate the errors of the centroid.