Photoionization models of the Eskimo nebula: evidence for a binary   central star?
  The ionizing star of the planetary nebula NGC 2392 is too cool to explain the high excitation of the nebular shell, and an additional ionizing source is necessary. We use photoionization modeling to estimate the temperature and luminosity of the putative companion. Our results show it is likely to be a very hot (Teff ~ 250kK), dense white dwarf. If the stars form a close binary, they may merge within a Hubble time, possibly producing a Type Ia supernova. 
Ashkbiz Danehkar, David J. Frew, Quentin A. Parker, Orsola De Marco
astro-ph.SR