The frequency content of Delta Sct stars as determined by photometry

The results obtained by means of the photometric approach to the study of Delta Sct stars are extensively discussed. The different frequency contents of the three best candidates for asteroseismological studies (FG Vir, 4 CVn and XX Pyx) are presented and compared; the importance of the amplitude variations and of the combination terms is emphasized. The analysis of other multiperiodic variables shows how a large variety of nonradial modes are excited; in some cases, modifications of the power spectrum can be observed over a few years and new modes can be seen to grow. Among monoperiodic pulsators, constant as well as variable amplitudes can be observed. The difficulty of identifying an oscillation yielding quantum numbers is emphasized; the possibilities offered by $\delta$ Sct stars belonging to binary systems and open clusters are discussed. In this respect, combining the photometric and the spectroscopic approaches could lead to a solution. A comparison is also made between low- and high-amplitude pulsators, finding similarities. The use of a reliable Period-Luminosity-Colour relationships toward the shortest periods can greatly help mode identification in the galactic stars; moreover, it could provide an independent verification of extragalactic distances.