A SURVEY FOR PULSATIONS IN O-VI NUCLEI OF PLANETARY-NEBULAE

We present results of a CCD survey for pulsational variability in a sa mple of 29 hot, hydrogen-deficient planetary-nebula nuclei (PNNs) with ''O VI'' (early WC) or ''PG 1159'' spectra. We identify six PNNs, tho se of NGC 246, 1501, 2371-2, 2867, 5189, and 6905, as low-amplitude pu lsating variables similar to K 1-16 and PG 1159-035 (GW Vir); this bri ngs to 14 the number of PNNs and isolated white dwarfs now known to be long to this class. Time-series analyses show that the PNN pulsators t end to have longer pulsational periods (11.5 to 31 min) than their GM Vir white-dwarf counterparts, but within the PNNs there is little corr elation between spectral subtype and pulsation period. Most (if not al l) 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 whit e-dwarf pulsators, whose power spectra generally vary only on lime sca les of years, The variable power spectra and low pulsation amplitudes suggest that the true fraction of pulsators among hot, hydrogen-defici ent PNNs may be higher than found in our survey. (C) 1996 American Ast ronomical Society.