Two patterns of correlated X-ray timing and spectral behaviour in low-mass X-ray binaries.

Exosat data are used to study the power spectra of 16 LMXBs as a function of their behavior in X-ray color-color diagrams. The study confirms the existence of a class of sources with three-branched Z-shaped colour-colour diagrams characterized by a temporal behavior that is strongly dependent on the position of the source on the Z. Evidence for an additional class of sources with a different pattern of correlated timing and spectral behavior is found and examined.