  The discovery of source states in the X-ray emission of black-hole binaries
and neutron-star low-mass X-ray binaries constituted a major step forward in
the understanding of the physics of accretion onto compact objects. While there
are numerous studies on the correlated timing and spectral variability of these
systems, very little work has been done on high-mass X-ray binaries, the third
major type of X-ray binaries. The main goal of this work is to investigate
whether Be accreting X-ray pulsars display source states and characterise those
states through their spectral and timing properties. We have made a systematic
study of the power spectra, energy spectra and X-ray hardness-intensity
diagrams of nine Be/X-ray pulsars. The evolution of the timing and spectral
parameters were monitored through changes over two orders of magnitude in
luminosity. We find that Be/X-ray pulsars trace two different branches in the
hardness-intensity diagram: the horizontal branch corresponds to a
low-intensity state of the source and it is characterised by fast colour and
spectral changes and high X-ray variability. The diagonal branch is a
high-intensity state that emerges when the X-ray luminosity exceeds a critical
limit. The photon index anticorrelates with X-ray flux in the horizontal branch
but correlates with it in the diagonal branch. The correlation between QPO
frequency and X-ray flux reported in some pulsars is also observed if the peak
frequency of the broad-band noise that accounts for the aperiodic variability
is used. The two branches may reflect two different accretion modes, depending
on whether the luminosity of the source is above or below a critical value.
This critical luminosity is mainly determined by the magnetic field strength,
hence it differs for different sources.
