Time-frequency diagram (bottom image: blue passband; top image: red passband) of the Type II frequency group around 1.55 c/d in η Cen, after pre-whitening for the 1.5562 c/d frequency. AOV spectra with a sampling of 0.0005 c/d were calculated over 3 d sliding averages. The frequency interval shown (abscissa) is 1.1–1.7 c/d. For both sub-images the AOV power (white is highest) was separately normalized to the respective mean power to facilitate comparison. The ordinates (in days) have arbitrary zero points. The two data strings have different starting dates but end at about the same time (cf. Fig. 4).