Upper panel: time evolution of the extinction at 5500 Å. The horizontal dashed line marks the best fit value. The large dots are the average values computed within the time intervals indicated by the horizontal bars. The vertical bars are the associated rms deviations. The time intervals include the same number of measurements (61), and the points are placed at the average time within each bin. Lower panel: distribution of the extinction coefficient at 4000 Å (left) and 6800 Å (right).