2006A&A...454..527G

We searched for photometric variability in a sample of hot, hydrogen-deficient planetary nebula nuclei (PNNi) with ``PG 1159'' or ``O VI'' spectral type, most of them embedded in a bipolar or elliptical planetary nebula envelope (PNe). These characteristics may indicate the presence of a hidden close companion and an evolution affected by episodes of interaction between them. We obtained time-series photometry from a sample of 11 candidates using the Nordic Optical Telescope (NOT) with the Andalucia Faint Object Spectrograph and Camera (ALFOSC), modified with our own control software to be able to observe in a high-speed multi-windowing mode. The data were reduced on-line with the real time photometry (RTP) code, which allowed us to detect periodic variable stars with small amplitudes from CCD data in real time. We studied the properties of the observed modulation frequencies to investigate their nature. We report the first detection of low-amplitude pulsations in the PNNi VV 47, NGC 6852, and Jn 1. In addition, we investigated the photometric variability of NGC 246. Time-series analysis shows that the power spectra of VV 47, NGC 6852, and NGC 246 are variable on time scales of hours. Power spectra from consecutive nights of VV 47 and NGC 6852 show significant peaks in different frequency regions. The same type of variability is present in NGC 246 in 2 observing runs separated by 3 days. Changes are also found in the power spectra of VV 47 and NGC 246 during the same night. The VV 47 power spectra are peculiar since they present modulation frequencies in a wide range from 175 to 7600µHz. This is different from the previously known pulsating PNNi where no frequencies are found above ∼3000µHz. The high-frequency modulation observed in VV 47 may be due to g-modes triggered by the ε-mechanism, observed for the first time.