A photometric and spectroscopic study of the brightest northern Cepheids — I. Observations

We present simultaneous UBV uvby photometry for the 18 brightest northern Cepheids carried out between 1995 and 1997. Additionally, two fainter stars have been observed in the Johnson system only. The whole photometric data base contains about 3500 individual data points for 20 stars. The accuracy has been carefully tested with different methods. A serious systematic difference has been found between the present data set and the Stro¨mgren photometry available in the literature, which has probably been caused by the peculiar filter set used in the earlier study.  As an extension to the photometry, we took high‐resolution optical spectra at David Dunlap Observatory in the red spectral region (λ/Δλ≈ 40 000, in the interval of 6200 and 6600 Å including Hα). The spectroscopic programme contained 12 stars from the photometric programme, the newly discovered bright classical Cepheid CK Cam and two double‐mode Cepheids (TU Cas and CO Aur). New radial velocities obtained with the cross‐correlation technique are presented. We found significant velocity differences between two cross‐correlated spectral regions (6188–6220 and 6405–6435 Å) as large as 0.8–1.2 km s −1 , which show very characteristic phase dependence in certain Cepheids.  Finally, recent period variations are briefly discussed in terms of phase jumps and duplicity.