  In the analysis of variable stars, the problem of sampling is central. This
article focusses on the determination of the Nyquist frequency. It is well
defined in the case of regular sampling. However, the time series of variable
stars observations are generally unevenly sampled. Fourier analysis using the
spectral window furnishes some clues about the equivalent Nyquist frequency in
the irregular case. Often it is pushed very high, and thus very short periods
can be detected. A specific example is shown, drawn from MACHO databases.
