Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations^*

II. Open cluster Platais 1 (C2128+488) and small-amplitude Cepheid V1726 Cygni

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; panchuk@sao.ru
⁴ Isaac Newton Institute of Chile, SAO RAS Branch, Russia

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

Received: 27 February 2001
Accepted: 24 July 2001

Abstract

The small-amplitude Cepheid V1726 Cyg and two members of open cluster Platais 1 (Platais 1 star No. 1 (1921) and Platais 1 star No. 111 (1600)) were investigated, using high-resolution CCD spectra. The following results were obtained: 1) All objects have the same metallicities, close to that of the Sun (for V1726 Cyg weighted average [ Fe/H, for Platais 1 star No. 1 (1921) [ Fe/H); 2) values of T_eff and for the B-stars are in excellent agreement with those determined from colour indices using a , ) calibration; 3) the elemental abundances indicate that V1726 Cyg is in the post first dredge-up stage with an age near , and is crossing the Cepheid instability strip for the third time. Mean values of K and permit us to refine its colour excess to , which for a distance of pc corresponds to . The Cepheid could therefore be pulsating in the fundamental tone, although pulsation in the first overtone is not excluded; 4) Platais 1 star No. 1 (1921) is a slowly rotating HgMn-star with a high helium content, while Platais 1 star No. 111 (1600) is a rapidly rotating main-sequence star with a helium content comparable to that of the Sun; 5) the age of the open cluster is estimated to be about yr.