The variability in the shape of P Cygni line profiles is important in connection with the mechanisms responsible for the wind production and for the constancy of the associated mass loss rates. Comprehensive studies of these changes have been made with the IUE satellite in population I OB stars, showing that essentially all the studied stars exhibit variability in their P Cygni profiles at various timescales from minutes to years (cf. Baade 1988; Henrichs 1988). Recently Kaper et al. (1996) have performed an extensive study of ten bright O stars covering a long time interval (1986 - 1992) with observations spanning time intervals from days to hours, using simultaneous IUE and ground based high resolution spectroscopy, photometry, and polarimetry. Particularly significant was the discovery that the Discrete Absorption Components (DAC) phenomenon is very general in population I O stars and practically ubiquitous. The accurate description of the dynamics of the DAC led the authors to stress the likely importance of the stellar rotation in influencing the structure of the winds.
On the other hand the problem of wind variability in hot evolved stars, particularly in CSPN, has received little attention so far. This in spite of the fact that the fast winds in CSPN are recognised, according to the multiple wind theory (Kwok et al. 1978; Kwok 1987; Khan 1989), to be essential for the formation and the evolution of the observed nebulae. The relevant information is contained in the SWP spectral range of the IUE satellite. A first effort in this direction was made by Patriarchi & Perinotto (1995, hereafter Paper I), who studied all the available good quality IUE high resolution SWP spectra (14 objects). Variations in P Cygni lines up to 50% in the shape of the profiles over time scales from months to years was revealed in four out of the fourteen observed objects, while in NGC 6543 photometric variability was suggested. Particularly remarkable was the lack of variations in the two pure WR stars contained in the sample of the Paper I (NGC 40 and BD +30 3639).
The database used in Paper I was, unfortunately, not so rich since in most cases only 2-3 epochs were covered.
With the present paper we contribute new high resolution IUE observations of NGC 40, NGC 6543, NGC 6826, and BD +30 3639. In Sect.2 (click here) the observational data and the method of analysis are presented, while the results are given in Sect.3 (click here). The discussion follows in Sect.4 (click here) .