The results of our second stage (1998-2018) of the detailed spectroscopy of peculiar supergiants identified with galactic infrared sources, performed mainly at the 6-meter BTA telescope are summarized. The main aspect of the program is a search for the evolutionary variations in the chemical composition of stars, past the AGB stage and the TDU, as well as an analysis of spectral manifestations of kinematic processes in their extended, often unstable, atmospheres and in the envelopes. The most significant result is detection of the s-process element excesses in seven single post-AGB stars, which confirms the theory of evolution of this type of stars. In three of these stars we for the first time discovered the ejection of the s-process heavy metals to the circumstellar envelopes. A lithium excess was found in the atmospheres of two peculiar supergiants V2324 Cyg and V4334 Sgr. The results of investigation of the kinematical state of atmospheres and envelopes will clarify the equilibrium of matter produced by stars in the AGB and post-AGB stages and delivered to the interstellar medium.
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