Physical conditions derived from OII recombination lines in planetary   nebulae and their implications
  Based on high quality observations of multiplet V1 of OII and the NLTE atomic computations for OII we study the density and temperature of a sample of PNe. We find that, in general, the densities derived from recombination lines of OII are similar than the densities derived from forbidden lines. This implies that the signature for oxygen rich clumps of high density and low temperature is absent in most of the objects of our sample. Electron pressures derived from the hotter zones are similar or slightly larger than those derived from the colder zones, suggesting the presence of shock waves. The average temperatures and t2 values derived from H, He and O lines are similar and consistent with chemical homogeneity. These results suggest that the abundances of these objects are the ones derived from recombination lines. 
Antonio Peimbert, Manuel Peimbert, Gloria Delgado-Inglada, Jorge   Garcia-Rojas, and Miriam Pe\~na
astro-ph.GA astro-ph.SR