A PHOTOMETRIC AND SPECTROSCOPIC STUDY OF THE BRIGHTEST NORTHERN CEPHEIDS - I - OBSERVATIONS

We present simultaneous UBV and uvby photometry for the 18 brightest n orthern 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 method s. A serious systematic difference has been found between the present data set and the Stromgren photometry available in the literature, whi ch has probably been caused by the peculiar filter set used in the ear lier study. As an extension to the photometry, we took high-resolution optical spectra at David Dunlap Observatory in the red spectral regio n (lambda/Delta lambda- 40000, in the interval of 6200 and 6600 Angstr om including Her). The spectroscopic programme contained 12 stars from the photometric programme, the newly discovered bright classical Ceph eid CK Cam and two double-mode Cepheids (TU Cas and CO Aur). New radia l velocities obtained with the cross-correlation technique are present ed. We found significant velocity differences between two cross correl ated spectral regions (6188-6220 and 6405-6435 Angstrom) as large as 0 .8-1.2 km s(-1), which show very characteristic phase dependence in ce rtain Cepheids. Finally, recent period variations are briefly discusse d in terms of phase jumps and duplicity.