2002A&A...395..929R

We present extensive new spectroscopy and imaging of PN G135.9+55.9. We use these data as constraints to photoionization models to derive limits on the oxygen abundance. We find that PN G135.9+55.9 has an oxygen abundance less than 1/50 of the solar value. Our models favour a value of 12+logO/H between 5.8 and 6.5dex, confirming that PN G135.9+55.9 is the most oxygen-poor planetary nebula known (Tovmassian et al., 2001A&A...370..456T]). We also derive Ne/O=0.5±0.3, S/O<0.094, and Ar/O<0.23. Although the value of Ne/O is nominally high, it need not imply that the progenitor of PN G135.9+55.9 converted any of its initial oxygen abundance to neon. The helium abundance appears to be very low, He/H∼0.08, but a precise determination will require a much more detailed study. We find that Hα/Hβ is lower than expected and perhaps variable, a finding for which we have no clear explanation.