  The aspects of the analysis of photometric time-series obtained on
double-mode or multiperiodic pulsating stars are briefly reviewed. In
particular, the ratios between frequencies are used to pin cases revealing
peculiarities. In addition to the Petersen diagrams, we also demonstrated that
the period ratios can detect interesting objects. In particular, new results
are obtained on High--Amplitude Delta Scuti contained in the OGLE-II database.
