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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 windactivity, which in turn can be studied through optical spectroscopicaldedicated observations. We study optical spectral features of the Becircumstellar disk to test their long-term variability and their relation withthe X-ray activity. Special attention has been given to the H$\alpha$ emissionline, that is one of the best tracers of the disk conditions. We obtainedoptical broadband medium resolution spectra from a dedicated campaign with theAnglo-Australian Telescope and the Southern African Large Telescope in2014-2015. Data span over one entire binary orbit, and cover both X-rayquiescent and moderately active periods. We used Balmer emission lines tofollow the evolution of the circumstellar disk. We observe prominent spectralfeatures, like double-peaked H$\alpha$ and H$\beta$ emission lines. TheH$\alpha$ V/R ratio significantly changes over a time scale of about one year.Our observations are consistent with a system observed at a large inclinationangle ($i\gtrsim60^{\circ}$). The derived circumstellar disk size shows thatthe disk evolves from a configuration that prevents accretion onto the neutronstar, to one that allows only moderate accretion. This is in agreement with thecontemporary observed X-ray activity. Our results are interpreted within thecontext of inefficient tidal truncation of the circumstellar disk, as expectedfor this source's binary configuration. We derived the H$\beta$-emitting regionsize, which results about half of the corresponding H$\alpha$-emitting disk,and constrain the luminosity class of V850 Cen as III-V, consistent with thepreviously proposed class.}

Optical spectroscopy of the Be/X-ray Binary V850 Cen/GX 304-1 during faint X-ray periodical activity