  The importance of Cepheids is well known in many fields of astronomy. Section
2 introduces the Fourier decomposition, a tool to describe quantitatively the
light curves of pulsating stars. Sect. 3 shows how an observational result was
progressively built on the basis of old and new data; the latter were collected
on selected targets just to clarify some controversial points. Since different
pulsation modes were suspected among Cepheids, an independent confirmation was
searched for. To do that we applied the least-squares technique to Double Mode
Cepheids. By obtaining a very satisfactory description of their pulsational
content (Sect.4), we could confirm that between Cepheids with P<8d they are
both fundamental and first overtone pulsators (Sect.5). The detection of small
amplitude cross-coupling terms and higher harmonics in the light curves of
Double Mode Cepheids allowed us to quantify the properties of the high-order
terms and hence to discover other peculiarities (Sect.6), very useful to
complete the scenario of the resonance effects and to test some theoretical
models (Sect.7). Therefore, we can study the Cepheids from the point of view of
asteroseismology since to find resonance effects is as to sound their
interiors.
