  A photoelectric monitoring program has been applied, during the last four
years, to five central stars of planetary nebulae (PNNs) with strong O\,VI
$\lambda$ 3811--34 \AA \hspace{0.1mm} emission. NGC\,6905 and, marginally,
NGC\,2452, show intrinsic luminosity variations, while NGC\,7026, IC\,2003 and
NGC\,1501 have constant luminosity within a few mmag. Photometric data have
been analyzed with the best available packages for power--spectra reductions.
Both pulsators have periods and physical characteristics well encompassed by
the theoretical pulsational models relative to these stars.
