Power density spectra based on the Fourier transform (Scargle 1982; Horne & Baliunas 1986) of X-ray light curves. The periodogram was calculated from ν = 1 /P to ν = N₀/P with a spacing P^-1, where N₀ is the number of data points. Various false alarm probability levels are marked. From left to right: 1) combined zero and first-order Chandra HETGS light curve of β Cru in hard (1.−10.0 keV) band where pulsations were suggested by Cohen et al. (2008). There is about a 50% probability that this period is spurious; 2) zero-order Chandra’s HETGS light curve of β Cru in full (0.2−10.0 keV) band, there is more than a 50% probability that this period is spurious; 3) EPIC PN light curve of ξ¹ CMa in hard band (0.7−10.0 keV).