  We have investigated the long-term X-ray variability, defined as the
root-mean-square (rms) of the ASM RXTE light curves, of a set of galactic
Be/X-ray binaries and searched for correlations with system parameters, such as
the spin period of the neutron star and the orbital period and eccentricity of
the binary. We find that systems with larger rms are those harbouring fast
rotating neutron stars, low eccentric and narrow orbits. These relationships
can be explained as the result of the truncation of the circumstellar disc. We
also present an updated version of the Halpha equivalent width-orbital period
diagram, including sources in the SMC. This diagram provides strong
observational evidence of the interaction of neutron star with the
circumstellar envelope of its massive companion.
