[WC] central stars of planetary nebula and the born-again phenomenon

[WC] central stars of planetary nebula are members of the larger class of hydrogen-deficient central stars. The whole class constitutes about 15% of all spectroscopically-known central stars. Observational connections between [WC] central stars and the born-again phenomenon show that at least a fraction of the [WC] stars can be created through this scenario. However, it is unlikely that the class as a whole evolved through this channel. In this paper the arguments against a born-again origin for the whole class of [WC] central stars of planetary nebula are outlined. It is suggested that the role of the H-deficient weak emission lines stars might be crucial in explaining the origin of [WC] stars. It is also demonstrated how difficult it is to pin down the exact stellar parameters of these objects (which help to position them on the HR diagram). This is due to the largely unknown distances and to the fact that small changes in the model assumptions can have large repercussions on the derived parameters. This difficulty hampers our efforts to determine the true evolutionary position of individual [WC] central stars, as well as their relationship to one another, and therefore pin down their origin.