Periodic behaviour of both 6.7 GHz sources. Panels in the leftmost column show the time series of spectral features. In the second column, grey points represent time series phased to the calculated period, red points show data binned with 0.1 phase intervals, and dashed black lines show the fitted asymmetric periodic power function. The third and fourth columns respectively show AoV and L−S periodograms. The dotted line in the last column represents the 1% false alarm probability level.