2001A&A...376..898M

We studied the long-term spectral and photometric behaviour of P Cygni over the 13.8 year interval from March 1985 to January 1999. The UBV photometry reveals a slow (∼7.4-year), low-amplitude (∼0.1mag) variation in V in which the star becomes redder when it brightens and vice versa. It underwent a possible maximum in the winter of 1985 (between JD 2446000 and JD 2446200), a minimum in the winter of 1989 (between JD 2447500 and JD 2447700), a maximum in the fall of 1992 (between JD 2448800 and JD 2449000), and a minimum in the spring of 1996 (between JD 2450100 and JD 2450300) The properties of this variation are typical for weak-active S Dor variables in ``short S Dor phase'' (van Genderen, 2001A&A...366..508V). Simultaneous spectroscopic observations show changes in the Hα equivalent width (corrected for the effect of the changing continuum) in positive correlation with the 7.4-year photometric oscillation in the V-band brightness. This result is interpreted as an indication that in P Cygni an increase in the stellar brightness, during the 7.4-year SD phase, is likely accompanied by an increase in the mass-loss rate. In this behaviour P Cygni is similar to S Dor and R71. From simple considerations, it is concluded that the 7.4-year SD cycle is probably a combination of an increasing radius/decreasing effective temperature and an expanding pseudo-photosphere.