We have conducted non-local thermodynamic equilibrium (LTE) and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. « lessĭY Cen has shown a steady fading of its visual light by about one magnitude in the last 40 yr, suggesting a secular increase in its effective temperature. The various indicators provide a consistent set of atmospheric parameters: T for a representative post-extended horizontal branch star. Defining atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including He i and He ii line profiles, and the ionization equilibrium of ion pairs such as C ii/C iii and N ii/N iii. We found that the relative abundance of is generally large in red symbiotic systems.Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. This was confirmed in a number of objects by the low 12C/ 13C ratio (5-23). The enrichment in 14N isotope, found in all these objects, indicates that the giants have experienced the first dredge-up. Our analysis reveals metallicities distributed in a wide range from slightly supersolar ( ∼ +0.35 dex) to significantly subsolar ( ∼ -0.8 dex) but principally with near-solar and slightly subsolar metallicity ( ∼ -0.4 to -0.3 dex). Spectrum synthesis methods employing standard local thermal equilibrium analysis and atmosphere models were used to obtain photospheric abundances of CNO and elements around the iron peak (Fe, Ti, Ni, and Sc). Here we present results for 24 S-type systems. This is the third in a series of papers on the chemical composition of symbiotic giants determined from high-resolution (R ∼ 50 000), near-infrared spectra. However, the number of symbiotic giants with fairly well determined photospheric composition is still insufficient for statistical analyses. The elemental abundances of symbiotic giants are essential to address the role of chemical composition in the evolution of symbiotic binaries, to map their parent population, and to trace their mass transfer history.
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