2001A&A...376..885U

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]=+0.05, for Platais 1 star No. 1 (1921) [Fe/H]=+0.13); 2) values of T_eff and logg for the B-stars are in excellent agreement with those determined from (B-V) colour indices using a (B-V)~(T_eff, logg) calibration; 3) the elemental abundances indicate that V1726 Cyg is in the post first dredge-up stage with an age near 1.5x10⁸, and is crossing the Cepheid instability strip for the third time. Mean values of T_eff=6100K and logg=2.35±0.05 permit us to refine its colour excess to E_B–V=0.33mag, which for a distance of d=1568±13pc corresponds to M_V=-2.99mag. 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 2.5x10⁸yr.