2 Observations and data reduction

Our data sample consists of 45 CCD spectra published by Stahl et al. (1993) together with a further 10 photographic spectra obtained with the coud'e spectrograph of the 2 m telescope at the National Astronomical Observatory (Bulgaria). The spectra cover the period from May to December 1990. The spectral resolution is about 27000 in the blue and 18000 in the red photographic region. The resolution of the CCD observations is a factor of two lower. Additional information about the observational material and its reduction can be found in Stahl et al. (1991) and Markova (1986b).