Fourier decomposition and frequency analysis of the pulsating stars with P < 1 d in the OGLE database .

The OGLE database is revisited to investigate in more detail the properties of the Fourier parameters. Methodological improvements led us to identify a clear separation among High–Amplitude δ Scuti (HADS), RRc and RRab stars. The bimodal distribution of the R21parameter in HADS stars is explained as a contamination effect from RRc stars: there is evidence that all stars with 0.20< P <0.25 d are RRc variables. The previously claimed existence of a subclass of unusual HADS is demonstrated to be a spurious result. Candidate overtone pulsators are found among HADS and RRc variables. The properties of the Fourier parameters are discussed as a function of the physical conditions in the stars involved. Among the field RRab stars we detected different light– curve groups producing distinct “tails” in the Fourier plots for P >0.55 d; evolutionary phases or the combination of different physical conditions (not only metallicity) are suggested to explain this separation, observed also in the cluster RRab stars. The stellar parameters of RRc stars in a given globular cluster show different tendencies than those of RRc stars from different clusters.