  The actual value of the oxygen abundance of the metal-poor planetary nebula
PN G135.9+55.9 has frequently been debated in the literature. We wanted to
clarify the situation by making an improved abundance determination based on a
study that includes both new accurate observations and new models. We made
observations using the method of integral field spectroscopy with the PMAS
instrument, and also used ultraviolet observations that were measured with
HST-STIS. In our interpretation of the reduced and calibrated spectrum we used
for the first time, recent radiation hydrodynamic models, which were calculated
with several setups of scaled values of mean Galactic disk planetary nebula
metallicities. For evolved planetary nebulae, such as PN G135.9+55.9, it turns
out that departures from thermal equilibrium can be significant, leading to
much lower electron temperatures, hence weaker emission in collisionally
excited lines. Based on our time-dependent hydrodynamic models and the observed
emission line [OIII]5007, we found a very low oxygen content of about 1/80 of
the mean Galactic disk value. This result is consistent with emission line
measurements in the ultraviolet wavelength range. The C/O and Ne/O ratios are
unusually high and similar to those of another halo object, BoBn-1.
