Wind variability in central stars of planetary nebulae. II.

New high resolution IUE spectra of central stars of the planetary nebulae (CSPN) NGC 40, NGC 6543, NGC 6826, and ED +30 3639 have been obtained between August 1994 and September 1995 to explore the variability of their P Cygni profiles. The data have been analyzed together with previous observations of the same stars. Significant changes in the profiles of several of the typical wind lines of the N V, O IV, O V, Si IV, C IV, and N IV ions have been revealed in all the observed stars. By combining these results with the ones obtained in a previous paper of this series (Patriarchi & Perinotto 1995; Paper I), it is found that half of the fourteen observed CSPN exhibit changes in the shape of their P Cygni profiles up to levels of 10 - 50% in timescales of years, analogous to similar timescale variations exhibited by hot stars of population I. They are: NGC 40, NGC 1535, NGC 2392, NGC 6543, NGC 6826, IC 4593 and BD + 30 3639. The ones for which changes have not been detected are: NGC 246, NGC 6210, NGC 6572, NGC 7009, IC 418, IC 2149 and Lo 8. The present paper completes the survey of this type (at least 2 - 3 well separated high resolution good exposures for each star) that could have been done in central stars of planetary nebulae using the IUE satellite. In a typical case (NGC 1535) we estimate that the changes in the physical properties of the wind, resulting from the observed changes in the P Cygni profiles, amount for (M)over dot q(NV) to a factor of 3 +/- 1.