The study of iron lines from 49 high-resolution IUE spectra observed in 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 absorption lines of Fe III often have two components while Fe II lines can have three. P Cygni is the only luminous blue variable 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 time. The disappearance of shell components observed in Si, O and N lines is possibly a new unique phenomenon.