  Be/X-ray binaries are the most populous class of High Mass X-ray Binaries.
Their X-ray duty cycle is tightly related to the optical companion wind
activity, which in turn can be studied through optical spectroscopical
dedicated observations. We study optical spectral features of the Be
circumstellar disk to test their long-term variability and their relation with
the X-ray activity. Special attention has been given to the H$\alpha$ emission
line, that is one of the best tracers of the disk conditions. We obtained
optical broadband medium resolution spectra from a dedicated campaign with the
Anglo-Australian Telescope and the Southern African Large Telescope in
2014-2015. Data span over one entire binary orbit, and cover both X-ray
quiescent and moderately active periods. We used Balmer emission lines to
follow the evolution of the circumstellar disk. We observe prominent spectral
features, like double-peaked H$\alpha$ and H$\beta$ emission lines. The
H$\alpha$ V/R ratio significantly changes over a time scale of about one year.
Our observations are consistent with a system observed at a large inclination
angle ($i\gtrsim60^{\circ}$). The derived circumstellar disk size shows that
the disk evolves from a configuration that prevents accretion onto the neutron
star, to one that allows only moderate accretion. This is in agreement with the
contemporary observed X-ray activity. Our results are interpreted within the
context of inefficient tidal truncation of the circumstellar disk, as expected
for this source's binary configuration. We derived the H$\beta$-emitting region
size, which results about half of the corresponding H$\alpha$-emitting disk,
and constrain the luminosity class of V850 Cen as III-V, consistent with the
previously proposed class.}
