Long-term Photometric and Spectral Study of Planetary Nebula Variability (1968-2005)

The results of long term observations of photometric ($UBV$) and spectral variability of several PNe are presented. For the most variable nebulae, IC 4997 and NGC 6572, showing both photometric and spectral changes in time, the main physical parameters ($T_e$, $N_e$ and $T_{*}$) and their time variations were estimated and discussed.