Long-term cross-correlation curves. The ZDCF is represented in black and its 1σ uncertainty by the grey contour. Coloured dotted lines correspond to different significance levels, from 1σ to 4σ. Left: cross-correlation between the optical and HE γ-ray light curves. Middle: cross-correlation between the radio and optical light curves. Right: cross-correlation between the radio and HE γ-ray light curves.