  We present Gemini-South observations of nine faint and extended planetary
nebulae. Using direct images taken with the spectrograph GMOS, we built the
$(u' - g')$ vs. $(g' - r')$ diagrams of the stars in the observed areas which
allowed us, also considering their geometrical positions, to identify the
probable central stars of the nebulae. Our stellar spectra of seven stars, also
taken with GMOS, indicate that four (and probably two more) objects are white
dwarfs of the DAO subtype. Moreover, the white dwarf status of the four stars
is confirmed by the parameters $ T_{\mathrm{eff}}$ and $ \log g$ derived with
the help of theoretical stellar spectra. Given this evidence, we propose that
these hot stars are the central ionizing sources of the nebulae. With this work
we hope to help improve the current scarce statistics on central white dwarfs
in planetary nebulae.
