Time dependence of the frequency (lower panel) and amplitude (upper panel) of the 1.5562-c/d variability in η Cen (red passband: +; blue passband: ×). Frequencies and amplitudes were determined as sliding averages over three-day intervals from sine fits. For comparison, the sine curve fitted to the light curve in Fig. 4 is overplotted with arbitrary scaling and vertical offset (note that the magnitude scale is inverted).