Planetary nebulae: Multi-wavelength probes of stellar and galactic evolution Proceedings IAU Symposium No. 323, 2017 X. Liu, L. Stanghellini & A. Karakas, eds. c  2017 International Astronomical Union DOI: 00.0000/X000000000000000X [WN] central stars of planetary nebulae H. Todt 1 , B. Miszalski 2 , J. A. Toal´ a 3,4 , and M. A. Guerrero 4 1 Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany email: htodt@astro.physik.uni-potsdam.de 2 South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa; Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, South Africa 3 Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), 10617 Taipei, Taiwan, Republic of China 4 Instituto de Astrof´ ısica de Andaluc´ ıa, IAA-CSIC, Glorieta de la Astronom´ ıa s/n, E-18008 Granada, Spain Abstract. While most of the low-mass stars stay hydrogen-rich on their surface throughout their evolution, a considerable fraction of white dwarfs as well as central stars of planetary nebulae have a hydrogen-deficient surface composition. The majority of these H-deficient central stars exhibit spectra very similar to massive Wolf-Rayet stars of the carbon sequence, i.e. with broad emission lines of carbon, helium, and oxygen. In analogy to the massive Wolf-Rayet stars, they are classified as [WC] stars. Their formation, which is relatively well understood, is thought to be the result of a (very) late thermal pulse of the helium burning shell. It is therefore surprising that some H-deficient central stars which have been found recently, e.g. IC4663 and Abell 48, exhibit spectra that resemble those of the massive Wolf-Rayet stars of the nitrogen sequence, i.e. with strong emission lines of nitrogen instead of carbon. This new type of central stars is therefore labelled [WN]. We present spectral analyses of these objects and discuss the status of further candidates as well as the evolutionary status and origin of the [WN] stars. Keywords. Stars: abundances – Stars: AGB and post-AGB – Stars: atmospheres – Stars: mass- loss – Stars: PN PB 8 – Stars: IC 4663 – Stars: Abell 48 – Stars: PMR 5 – Stars: Wolf-Rayet 1. Introduction Wolf-Rayet central stars are hydrogen deficient central stars of planetary nebulae which exhibit in their spectra strong emission lines of helium, carbon and oxygen. Because their spectra resemble those of massive WC stars, they are called [WC] stars, with brackets to distinguish them from their massive counterparts. In spite of spectral similarities and comparable chemical composition, the formation of the low-mass [WC] stars is completely different from the formation of the massive WC stars. Stellar evolutionary models ac- counting for simultaneous burning and mixing explain the formation of a [WC] star by the occurrence of a thermal pulse (TP) at the very end or after the asymptotic giant branch (AGB) phase of a H-normal low-mass star. These models predict a hydrogen- deficient surface composition with carbon enriched up to X C = 40% after a late or very late TP (Althaus et al. 2005, Herwig 2001). Only in the case of a very late TP (VLTP) a supersolar nitrogen abundance of about X N = 1% is expected, but without any re- maining hydrogen. Thus low-mass central stars with WN-like surface abundances are theoretically not expected. 1 arXiv:1612.03719v1 [astro-ph.SR] 12 Dec 2016