Spectroscopic investigations of classical Cepheids and main-sequence starsin galactic open clusters and associations - II. Open cluster Platais 1 (C2128+488) and small-amplitude Cepheid V1726 Cygni

The small-amplitude Cepheid V1726 Cyg and two members of open cluster Plata is 1 (Platais 1 star No. 1 (1921) and Platais 1 star No. 111 (1600)) were i nvestigated, 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] = +0.05, for Platais 1 star No. 1 (1921) [Fe/H] = +0.13); 2) values of T-eff and log g for the B-stars are in excellent agreement with those determined from (B-V) colour indices usin g a (B-V) similar to (T-eff, log g) calibration; 3) the elemental abundance s indicate that V1726 Cyg is in the post first dredge-up stage with an age near 1.5 x 10(8), and is crossing the Cepheid instability strip for the thi rd time. Mean values of T-eff = 6100 K and log g = 2.35 +/- 0.05 permit us to refine its colour excess to EB-V = 0.(m)33, which for a distance of d = 1568 +/- 13 pc corresponds to M-V = 2.(m)99. The Cepheid could therefore be pulsating in the fundamental tone, although pulsation in the first overton e 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 tha t of the Sun; 5) the age of the open cluster is estimated to be about 2.5 x 10(8) yr.