FN Aquilae -an unusual Cepheid with anomalous CNO abundances

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 Te = 5483 K; log g =1 :25 and Vt =4 : 0k m 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 ((C=H) = 0:93 dex) and nitrogen ((N=H) = +0:09 dex) abundances, which is unusual for a Cepheid not crossing the instability strip for the rst time. Since FN Aql is an IRAS object and candidate protoplanetary nebulae according to Volk & Kwok (1989), 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.