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We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period December 2017 - January 2020 (MJD~58095 - MJD~58865). In December 2017 the $Halpha$, $Hbeta$, and HeI 6678 emission lines were symmetric with violet-to-red peak ratio $V/R approx 1$. During the first part of the period (December 2017 - August 2018) the V/R-ratio decreased to 0.5 and the asymmetry developed simultaneously in all three lines. In September 2018, a third component with velocity $approx 250$~km~s$^{-1}$ appeared on the red side of the HeI line profile. Later this component emerged in $Hbeta$, accompanied by the appearance of a red shoulder in $Halpha$. Assuming that it is due to an eccentric wave in the circumstellar disc, we find that the eccentric wave appeared first in the innermost part of the disc, it spreads out with outflowing velocity $v_{wave} approx 1.1 pm 0.2 $~km~s$^{-1}$, and the eccentricity of the eccentric wave is $e_{wave} approx 0.29 pm 0.07$. A detailed understanding of the origin of such eccentricities would have applications to a wide range of systems from planetary rings to AGNs.
During normal Type I outbursts, the pulse profiles of Be/X-ray binary pulsars are found to be complex in soft X-ray energy ranges. The profiles in soft X-ray energy ranges are characterized by the presence of narrow absorption dips or dip-like featur
The main goal of this work is to perform a detailed study of the optical variability of the Be/X-ray binary SAX J2239.3+6116. We obtained multi-colour BVRI photometry and polarimetry and 4000-7000 A spectroscopy. The optical counterpart to SAX J2239.
We present results obtained from an extensive near-infrared spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its ~111 day orbital period. This observation campaign is a part of the monitori
In this paper we study the timing and spectral properties of Be/X-ray binary pulsar EXO 2030+375 using a $Suzaku$ observation on 2012 May 23, during a less intense Type I outburst. Pulsations were clearly detected in the X-ray light curves at a baryc
We report the timing and spectral properties of Be/X-ray binary pulsar GX 304-1 by using two Suzaku observations during its 2010 August and 2012 January X-ray outbursts. Pulsations at ~275 s were clearly detected in the light curves from both the obs