  Rich regular frequency patterns were found in the Fourier spectra of
low-amplitude delta Scuti stars observed by CoRoT satellite (see Papar\'o et
al. 2016a,b). The CoRoT observations are, however, influenced by the disturbing
effect of the South Atlantic Anomaly. The effect is marginal for high amplitude
variable stars but it could be critical in the case of low amplitude variables,
especially if the frequency range of the intrinsic variation overlaps the
interval of the instrumental frequencies. Some tests were carried out both on
synthetic and real data for distinguishing technical and stars' frequencies.
