P Cygni in a short S Doradus phase. Spectroscopic and photometric evidences

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 (similar to 7.4-year), low-amplitude (similar to 0.1 mag) v ariation in V in which the star becomes redder when it brightens and vice v ersa. It underwent a possible maximum in the winter of 1985 (between JD 2 4 46 000 and JD 2 446 200), a minimum in the winter of 1989 (between JD 2 447 500 and JD 2 447 700), a maximum in the fall of 1992 (between JD 2 448 800 and JD 2 449 000), and a minimum in the spring of 1996 (between JD 2 450 1 00 and JD 2 450 300) The properties of this variation are typical for weak- active S Dor variables in "short S Dor phase" (van Genderen 2001). Simultan eous spectroscopic observations show changes in the H alpha equivalent widt h (corrected for the effect of the changing continuum) in positive correlat ion with the 7.4-year photometric oscillation in the V-band brightness. Thi s 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-ye ar SD cycle is probably a combination of an increasing radius/decreasing ef fective temperature and an expanding pseudo-photosphere.