Kovtyukh, V.Yegorova, I.Andrievsky, S.Korotin, S.Saviane, I.Lemasle, B.Chekhonadskikh, F.Belik, S.2019-05-282019-05-282018-06Monthly Notices of the Royal Astronomical Society, 477(2), pp. 2276-2285.0035-8711DOI: 10.1093/mnras/sty671http://repositorio.unab.cl/xmlui/handle/ria/8924Indexación: Scopus.We are grateful to George Wallerstein for valuable discussions and comments and for kindly provided spectra from APO. The authors thank the anonymous referee for her/his very valuable comments that significantly improved the paper. I.Y. thanks project ALMA-CONICYT 31150029 for support. B.L. acknowledges support from Sonderforschungsbereich SFB 881 ‘The Milky Way System’ (subproject A5) of the German Research Foundation (DFG). V.K., S.A., and S.K. acknowledge SCOPES grant no. IZ73Z0-152485 for financial support. V.K. and S.A. kindly acknowledge financial support from the Kenilworth Fund obtained through CRDF.The chemical composition of 28 Population II Cepheids and one RR Lyrae variable has been studied using high-resolution spectra. The chemical composition of W Vir variable stars (with periods longer than 8 d) is typical for the halo and thick disc stars. However, the chemical composition of BL Her variables (with periods of 0.8-4 d) is drastically different, although it does not differ essentially from that of the stars belonging to globular clusters. In particular, the sodium overabundance ([Na/Fe] ≈ 0.4) is reported for most of these stars, and the Na-O anticorrelation is also possible. The evolutionary tracks for BL Her variables (with a progenitor mass value of 0.8 solar masses) indicate that mostly helium-overabundant stars (Y = 0.30-0.35) can fall into the instability strip region. We suppose that it is the helium overabundance that accounts not only for the existence of BL Her variable stars but also for the observed abnormalities in the chemical composition of this small group of pulsating variables. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.enStars: AbundancesStars: BL Her typeStars: Type II CepheidsStars: W Vir typeArtículo