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Results of selected stellar spectroscopy programs at the 6-m telescope of SAO RAS performed with the NES echelle spectrograph

Over the past two decades the echelle spectrograph NES of the 6-m telescope was used to perform high resolution spectroscopy of far evolved stars spanning a wide range of initial masses. The studies cover a diversity of stars with high mass-loss rates during the preceding and current stages of evolution. All these stars have extended atmospheres and structured circumstellar envelopes produced by strong stellar winds. We have studied luminous blue variables near the Eddington limit; hot supergiants with B[e] phenomenon in spectra, which are very likely intermediate-mass binary systems soon after the fast mass exchange stage; a group of yellow hypergiants, as well as an extensive sample of low-mass post-AGB supergiants. The diverse nature of the types of these stars whose common feature is the presence of a circumstellar envelope makes the spectroscopy of such objects a comprehensive task. Such studies consist of many etapes, which include not only determining the peculiarities of their atmospheric chemical composition and understanding the role of supergiants in the enrichment of the interstellar medium with freshly synthesized elements, but also the determination of the evolution status of the objects considered, as well as search for and analysis of spectroscopic manifestations of kinematic processes in their extended and unstable atmospheres and gas-dust envelopes. We spectroscopically monitored selected objects to study in detail the instability of the kinematic state of the atmospheres of the stars considered. This review reports briefly the most significant observational obtained within the framework of the programs in 1998--2021.