  Context. At least 492 central stars of Galactic planetary nebulae (CSPNs)
have been assigned spectral types. Since many CSPNs are faint, these
classification efforts are frequently made at low spectral resolution. However,
the stellar Balmer absorption lines are contaminated with nebular emission;
therefore in many cases a low-resolution spectrum does not enable the
determination of the H abundance in the CSPN photosphere. Whether or not the
photosphere is H deficient is arguably the most important fact we should expect
to extract from the CSPN spectrum, and should be the basis for an adequate
spectral classification system.
  Aims. Our purpose is to provide accurate spectral classifications and
contribute to the knowledge of central stars of planetary nebulae and stellar
evolution.
  Methods. We have obtained and studied higher quality spectra of CSPNs
described in the literature as weak emission-line star (WELS). We provide
descriptions of 19 CSPN spectra. These stars had been previously classified at
low spectral resolution. We used medium-resolution spectra taken with the
Gemini Multi-Object Spectrograph (GMOS). We provide spectral types in the
Morgan-Keenan (MK) system whenever possible.
  Results. Twelve stars in our sample appear to have normal H rich photospheric
abundances, and five stars remain unclassified. The rest (two) are most
probably H deficient. Of all central stars described by other authors as WELS,
we find that at least 26% of them are, in fact, H rich O stars, and at least 3%
are H deficient. This supports the suggestion that the denomination WELS should
not be taken as a spectral type, because, as a WELS based on low-resolution
spectra, it cannot provide enough information about the photospheric H
abundance.
