SIMBAD references

We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period 1999-2018. Using new and published data, we found that during "disc-rise" the expansion velocity of the circumstellar disc is 0.4-0.7km/s. Our results suggest that the disc radius in recent decades show evidence of resonant truncation of the disc by resonances 10:1, 3:1, and 2:1, while the maximum disc size is larger than the Roche lobe of the primary and smaller than the closest approach of the neutron star. We find correlation between equivalent width of Hα emission line (Wα) and the X-ray flux, which is visible when 15Å<Wα≤40Å. The correlation is probably due to wind Roche lobe overflow.