G. Israelian et al., THE VARIABLE MASS-LOSS OF THE PECULIAR SUPERGIANT P-CYGNI, Monthly Notices of the Royal Astronomical Society, 283(1), 1996, pp. 119-136
The study of iron lines from 49 high-resolution IUE spectra observed i
n the period of 1985-1991 allowed us to find a repetition time between
two successive shells of the order of six months. We found that absor
ption lines of Fe III often have two components while Fe II lines can
have three. PCygni is the only luminous blue variable (LBV) that does
not show photometric variability on the scale of 1-2 mag and the only
one whose shell phenomena have been followed for some considerable tim
e, The recent disappearance of shell components observed in Si, O and
N lines during the last decades is possibly a new unique phenomenon.