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Published online by Cambridge University Press: 15 December 2006
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.