  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 R21 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.
