@article{PenaStastinskaEstebanetal.1999,
  author    = {Pena, M. and Stastinska, G. and Esteban, C. and Koesterke, Lars and Medina, S. and Kingsburgh, R.},
  title     = {Spectroscopy of planetary nebulae with [WR] nuclei},
  year      = {1999},
  language  = {en}
}
@article{MesaDelgadoEstebanGarciaRojas2015,
  author    = {Mesa-Delgado, A. and Esteban, C. and Garc{\´i}a-Rojas, J.},
  title     = {Ring Nebulae},
  series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  journal   = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88299},
  pages     = {325 -- 328},
  year      = {2015},
  abstract  = {Preliminary results are presented from spectroscopic data in the optical range of the Galactic ring nebulae NGC 6888, G2:4+1:4, RCW 58 and Sh2-308. Deep observations with long exposure times were carried out at the 6.5m Clay Telescope and at the 10.4m Gran Telescopio Canarias. In NGC 6888, recombination lines of C ii, O ii and N ii are detected with signal-to-noise ratios higher than 8. The chemical content of NGC 6888 is discussed within the chemical enrichment predicted by evolution models of massive stars. For all nebulae, a forthcoming work will content in-depth details about observations, analysis and final results (Esteban et al. 2015, in prep.).},
  language  = {en}
}
@article{PenaStastinskaEstebanetal.1998,
  author    = {Pena, M. and Stastinska, G. and Esteban, C. and Koesterke, Lars and Medina, S. and Kingsburgh, R.},
  title     = {Galactic planetary nebulae with Wolf-Rayet nuclei : I. Objects with [WC]-early type stars},
  year      = {1998},
  language  = {en}
}