A survey for pulsations in O VI nuclei of planetary nebulae

We present results of a CCD survey for pulsational variability in a sample of 29 hot, hydrogen-deficient planetary-nebula nuclei (PNNs) with "O VI" (early WC) or "PG 1159" spectra. We identify six PNNs, those of NGC 246, 1501, 2371-2, 2867, 5189, and 6905, as low-amplitude pulsating variables similar to K 1-16 and PG 1159-035 (GW Vir); this brings to 14 the number of PNNs and isolated white dwarfs now known to belong to this class. Time-series analyses show that the PNN pulsators tend to have longer pulsational periods (11.5 to 31 min) than their GW Vir white-dwarf counterparts, but within the PNNs there is little correlation between spectral subtype and pulsation period. Most (if not all) of the PNN pulsators have power spectra that are highly variable on time scales of months or less; this is in contrast to the GW Vir white-dwarf pulsators, whose power spectra generally vary only on time scales of years. The variable power spectra and low pulsation amplitudes suggest that the true fraction of pulsators among hot, hydrogen-deficient PNNs may be higher than found in our survey.