Test results for detection of the temporal variation of low-amplitude frequencies. Left panel: the amplitude of the constant, 3 mmag amplitude frequency injected at f_test = 3.0 d^-1 into the light curve of 241, a CoRoT RRc star (see Sect. 4.3). Right panel: a more complex case: we added a constant (3 mmag) amplitude frequency to the light curve of the strongly modulated Blazhko RR Lyrae, 962. We chose f_test = 3.4 d^-1. Horizontal lines show the amplitude of the injected signal, points with error bars come from our method. The upper row shows the shape and the vicinity of the recovered frequency in the applied eight bins.