Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon III. HD 50138

Context. B[e] stars are anomalous objects around which extended circumstellar matter is present. The observed properties of the central star are significantly affected by the surrounding material. Therefore, the use of standard synthetic spectra is disputable in this case and our capability to study these objects is limited. One of the possibilities is to analyse variations of the spectral features. Long-term spectroscopic observations are required for this, but are not found in the literature. For our study we choose the B[e] star HD 50138 of the FS CMa type because of the indication that this star is a post-main-sequence star, although still not highly evolved. Therefore, it can be a good object for testing evolutionary models. Currently, HD 50138 is the most extensively observed FS CMa star which makes it an ideal object for modelling. Our observations fill the gap in the available data.Aims. To describe the variability of HD 50138 we have monitored this star spectroscopically over the last twenty years. To search for the periodicity on short-term scales, series of night-to-night observations were also obtained. We were able to obtain 130 spectra from four different telescopes – 1.06 m at Ritter Observatory (échelle, R ~ 26 000, 32 spectra, 1994-2003), the Perek 2 m telescope at Ondrejov Observatory (slit, R ~ 12 500, 56 spectra, 2004-2013), the 2.12 m telescope at Observatorio Astronomico Nacional San Pedro Martir (échelle, R ~ 18 000, 16 spectra, 2005-2013), and the 0.81 m telescope at Three College Observatory (échelle, R ~ 12 000, 26 spectra, 2013-2014).Methods. We describe and analyse variations of the chosen lines. The measurements of the equivalent widths and radial velocities of the Ha, Hß, and [O?i] ?? 6300, 6364 Å lines are presented. The set of obtained spectra allows us to describe the changes on timescales from days to years.Results. The long-term quasi-periodic trend was found in the variations of the Ha equivalent width and confirmed by time dependent studies of the relative flux and equivalent width of the [O?i] 6300 Å line and radial velocity of the Ha violet peak. The two long periods of 3000 ± 500and 5000 ± 1000 days were detected there. We were able to catch moving humps in the Ha line, which reveal the rotating media around the star. An analysis of the correlation of equivalent widths, radial velocities, relative fluxes, and V/R ratios for various lines is presented in detail.Conclusions. We describe the spectral variability of HD 50138 over the last twenty years. Based on these data, we determine new restrictions for future modelling. We confirm the quasi-periodic behaviour of the object’s spectral variability, which probably reflects mass transfer in a binary system. This behaviour also supports a recently introduced explanation of the nature of FS CMa stars as post-merger systems