a) Evolution of the rotating pattern of M 2–9 from 1999 to 2010 in the images from the NOT telescope. [O iii] and Hα+[N ii] images are shown in the upper and lower rows, respectively. All images are displayed in a linear intensity scale. North is up and east to the left. The f.o.v. of each box is . b) Colour animation obtained combining images from 1997 to 2010. Green is the [O iii] emission, red is Hα+[N ii]. This figure is only available in the electronic version of the paper.