  We present the results of our monitoring program to study the long-term
variability of the Halpha line in high-mass X-ray binaries. We have carried out
the most complete optical spectroscopic study of the global properties of
high-mass X-ray binaries so far with the analysis of more than 1100 spectra of
20 sources. Our aim is to characterise the optical variability timescales and
study the interaction between the neutron star and the accreting material. Our
results can be summarised as follows: i) we find that Be/X-ray binaries with
narrow orbits are more variable than systems with long orbital periods, ii) we
show that a Keplerian distribution of the gas particles provides a good
description of the disks in Be/X-ray binaries, as it does in classical Be
stars, iii) a decrease in the Halpha equivalent width is generally observed
after major X-ray outbursts, iv) we confirm that the Halpha equivalent width
correlates with disk radius, v) while systems with supergiant companions
display, multi-structured profiles, most of the Be/X-ray binaries show at some
epoch double-peak asymmetric profiles, indicating that density inhomogeneities
is a common property in the disk of Be/X-ray binaries, vi) the profile
variability (V/R ratio) timescales are shorter and the Halpha equivalent width
are smaller in Be/X-ray binaries than in isolated Be stars, and vii) we provide
new evidence that the disk in Be/X-ray binaries is on average denser than in
classical Be stars.
