P Cygni is subject to at least three very-well documented types of instabilities during the last two decades of the 20th century: a short-SD cyclicity (<10 y), albeit a very weak active one and the well-known two types of microvariations. Considering the small variations in the scattered observations until the beginning of the 18th century and even further back in time, led de Groot et al. (2001) to speculate that the SD variability is going on since the eruption in the 17th century.
The 
Cygni-type microvariations show a remarkably stable quasi-period of 17
3; the other type of microvariation shows the typical 100-d cyclicity with usually redder colours in the maxima. We speculate that the DACs with a time scale of months and the recurrent
shell ejections, together with the photometric 100 d-type variations, are
related with each other, because
they all have roughly the same time scale. The 100 d-type light variations can then be explained by the variations of the physical parameters of the star while the colours are influenced by opacity changes in the wind due to recurrent shells. Well-observed S Dor variables with large amplitudes show near minimum brightness the Cygni-type, and near maximum the 100 d-type oscillations. Halfway between the two extrema the one is quickly replaced by the other, probably within a few months (e.g. van Genderen et al. 1990, 1997b). Both can only be seen simultaneously during this short transition stage. Therefore, the continuous presence of the latter two in the case of P Cyg is quite exceptional.
Acknowledgements
CS acknowledges a research grant from the Belgian Fund for Scientific Research (NFWO). Research at the Armagh Observatory is grant-aided by the Department of Culture, Arts and Leisure for Northern Ireland, and by the UK PPARC through the provision of the STARLINK network.
Copyright ESO 2001