In the following two subsections we describe (a) the technique
deviced to detect significant peaks in
presence of ``colored'' noise (Israel 1992; Israel & Stella 1996;
Stella et al. 1996), and (b) its generalisation to include a previously
developed technique for the analysis of power spectra from unevenly spaced
data (Lomb 1975; Scargle 1982;
Press & Rybicki 1989). Both techniques were adapted (§
and
§) to be successfully used for the analysis of ROSAT data.