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The suspected rapid rate of evolution of the central stars of planetary nebulae (Shklovsky, 1956) and the discovery by Seaton (1966) of their evolutionary track in the H-R diagram, have led to renewed interest in these stars. According to Seaton the evolutionary track starts at an atmospheric temperature of about 30000°K and a luminosity of 100 L ⊙. A maximum brightness is reached of 25000 L ⊙at T ∗ = 70000°K. Then T ∗ increases to 100000°K, at approximately constant L, whereafter, at this temperature, L decreases finally below 100L ⊙. This whole evolution is believed to take place in only 5 × 104 years. At such a rapid evolution, short-period fluctuations in brightness are expected and it should even be possible to detect secular variations in a lifetime. Aller and Liller (1957) discovered the variability of the spectrum of IC 4997 and Vorontsov-Velyaminov (1961) found variations in line intensities of NGC 6905. Khromov (1962) tried to explain the variations in both nebulae as a result of temperature changes of the central stars. Variations in the emission spectrum of IC 4997 were explained by Aller and Liller (1966) as a result of nebular expansion.
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