FN Aquilae -an unusual Cepheid with anomalous CNO abundances^*

¹ Astronomical Observatory of Odessa State University, Odessa 65014, Ukraine e-mail: val@deneb.odessa.ua
² Isaac Newton Institute of Chile, Odessa Branch, Ukraine
³ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhny Arkhyz, Stavropol Territory 369167, Russia e-mail: valenta@sao.ru
⁴ Isaac Newton Institute of Chile, SAO RAS Branch, Russia

Corresponding author: I. A. Usenko, igus@deneb.odessa.ua

Received: 23 May 2001
Accepted: 23 July 2001

Abstract

Spectroscopic analysis of the classical Cepheid FN Aql from three high-resolution CCD spectra near minimum and maximum light gives the following results: 1) Mean K; log and km s^-1; 2) Metallicities derived for each phase are consistent with each other and close to the solar value; 3) The Cepheid is crossing the instability strip for at least the third time. 4) It has anomalously low carbon ( dex) and nitrogen ( dex) abundances, which is unusual for a Cepheid not crossing the instability strip for the first time. Since FN Aql is an IRAS object and candidate protoplanetary nebulae according to Volk & Kwok ([CITE]), we assume that the star's peculiarities are caused either by envelope loss after the second dredge-up or that it is an unusual Cepheid for which existing nucleosynthesis and mixing models are unable to reproduce the CNO abundance pattern.