  Combination frequencies are not solutions of the perturbed stellar structure
equations. In dense power spectra from a light curve of a given multi-periodic
pulsating star, they can compromise the mode identification in an asteroseismic
analysis, hence they must be treated as spurious frequencies and conveniently
removed. In this paper, a method based on fitting the set of frequencies that
best describe a general non-linear model, like the Volterra series, is
presented. The method allows to extract these frequencies from the power
spectrum, so helping to improve the frequency analysis enabling hidden
frequencies to emerge from the initially considered as noise. Moreover, the
method yields frequencies with uncertainties several orders of magnitude
smaller than the Rayleigh dispersion, usually taken as the present error in a
standard frequency analysis. Furthermore, it is compatible with the classical
counting cycles method, the so-called O-C method, which is valid only for
mono-periodic stars. The method opens the possibility to characterise the
non-linear behaviour of a given pulsating star by studying in detail the
complex generalised transfer functions.
