Weak emission line [WELS] central stars of planetary nebulae are [WC]-PG1159 stars

The characteristics of the spectra of weak emission line stars (WELS) are found to be very similar to that of [WC]-PG1159 and PG1159 objects. On the basis of the spectra we find 31 WELS central stars of planetary nebulae to be in fact [WC]-PG1159 stars. These stars mark the transition from the [WC] central stars of planetary nebulae to the PG1159 (pre-)white dwarfs. The evolutionary sequence appears to be [WC late] [WC early] - WELS = [WC]-PG1159 - PG1159. The presence of nebulae and dust shells around these stars and also the significant percentage of [WC]-PG1159 stars among the [WC] central stars of PN indicates that late He-flash scenario may not be the principal mechanism for their post-AGB evolution towards the non-DA white dwarf stage. We found 4 hybrid central stars. They display a HeII/CIV 4650-4686 Angstrom trough very similar to that of PG1159 stars. However, their spectra show stellar Balmer lines indicating that they are relatively hydrogen-rich in contrast to the PG1159 stars.