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 \delta Scuti ( HADS ) , RRc and RRab stars .
The bimodal distribution of the R _ { 21 } parameter 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 .