The spectra were obtained in the coudé-spectrograph of the 2-m telescope at Rozhen Observatory (Bulgaria) with typical exposure times of 2-3 hours. Hydrogen treated, 103aO emulsion was used. Each plate covered a spectral interval from 3600Å to 4950Å and had a spectral resolution of about 0.35Å. Iron-argon hollow cathode lamp spectrum was obtained for wavelength calibration and the photometric calibration was achieved by 10-steps sensitometer exposures on each plate. The typical S/N ratio on the local continuum level was 10-15.
The spectra were digitized by MDM6 Joyce Loebl microdensitometer, with
20
wide slit and 10
step. We used the software system
ReWiA (Borkowski 1988) to process the data. The spectra were
linearized using dispersion curves of not less than 
0.05Å\
accuracy.
Figure 1: The evolution of 
over the whole period of
observation. Each observing season is shown in a separate
panel. In the upper part of the rightmost panel an
additional picture is inserted to define the different
types of Balmer absorption components - SSBA (strong
shifted Balmer absorptions) and SBA (slowest Balmer
absorptions)
All the spectra shown in Figs. 1 (click here), 2 (click here) and 3 (click here) are normalized to the local continuum level and shifted for plot clarity.
In Tables 1 and 3 and Figs. 4 (click here) and 5 (click here) single measurements of
equivalent widths and peak intensities of some spectral features are
presented. The accuracy of these measurements depends on two reasons
mainly: i) the typical errors associated with the photographic
emulsion which can be up to 
and ii) the difficulties arisen
during the continuum level fitting procedure. We can assume a value
of about 20% as an upper limit for the errors in these single
measurements.
Figure 2: The spectral region around 
, CaIIK and 
in
the spectrum of MWC560. Each observing season is represented
by two spectra. The horizontal and vertical marks show the
different absorption lines and their components. The
negative numbers on the right side represent the radial
velocity of the absorptions with the highest shifts in each
spectrum