Ultraviolet observations of planetary nebulae. III. Variability of the central star.

UV photometric observations of 39 planetary nebulae performed with the Astronomical Netherlands Satellite have been searched for variability of the central stars. No photometric variations of more than about 15% are found, but detectable variations are observed in the nuclei of five objects: IC 418, NGC 6543, A78, He2-131, and V-V 1-7. The results indicate that: (1) the only definite variations occurred in emission-line central stars with Of characteristics; (2) the variations in A78, NGC 6543, and He2-131 may be entirely spectroscopic in origin; (3) the variations in IC 418 appear to be both spectroscopic and photometric; and (4) V-V 1-7 is probably not a planetary nebula.