Optical R-band light curve (top) and radio 15 GHz light curve (middle) of Mkn 180. The solid blue line in middle panel shows the polynomial fit to radio data, which is subtracted from the optical light curve to estimate the contribution of the slowly varying component to optical flux. The bottom panel shows the results of the DCF study; the green, blue and red lines representing the 1σ, 2σ and 3σ significance limits, respectively.