Towards an improvement in the spectral description of central stars of
  planetary nebulae
  Context. There are more than 3000 known Galactic planetary nebulae (PNe), but
only 492 central stars of Galactic planetary nebulae (CSPN) have known spectral
types. It is vital to increase this number in order to have reliable
statistics, which will lead to an increase of our understanding of these
amazing objects.
  Aims. We aim to contribute to the knowledge of central stars of planetary
nebulae and stellar evolution.
  Methods. This observational study is based on Gemini Multi-Object
Spectrographs (GMOS) and with the Intermediate Dispersion Spectrograph (IDS) at
the Isaac Newton Telescope (INT) spectra of 78 CSPN. The objects were selected
because they did not have any previous classification, or the present
classification is ambiguous. These new high quality spectra allowed us to
identify the key stellar lines for determining spectral classification in the
Morgan-Keenan (MK) system.
  Results. We have acquired optical spectra of a large sample of CSPN. From the
observed targets, 50 are classified here for the first time while for 28 the
existing classifications have been improved. In seven objects we have
identified a P-Cygni profile at the He i lines. Six of these CSPN are late
O-type. The vast majority of the stars in the sample exhibit an absorption-type
spectrum, and in one case we have found wide emission lines typical of [WR]
stars. We give a complementary, and preliminary, classification criterion to
obtain the sub-type of the O(H)-type CSPN. Finally, we give a more realistic
value of the proportion of CSPN that are rich or poor in hydrogen.
