1996A&A...310..613R

We present a spectral analysis of the planetary nebula LoTr4 and its exciting star and report the discovery of a multiple-shell nebula structure. Two clearly separated shells can be detected around the central star with radii of 7.5" and 13.5" with a strong decrease of brightness from south-east to north-west. Relative to these shells the central star appears to be off-centered by about 2" to the south-east. A fragment of a third shell is found with a radius of 22.5" south-east of the central star. This and the high degree of asymmetry indicate that the nebula ejection was a complicated process. LoTr4 is of high excitation with a geometrical radius of r=0.6^+0.2_–0.3pc and a mass of M=0.29M_☉. Its helium abundance is definitely solar (±0.05dex), while the trace elements could also have slightly different abundances (±0.1dex). A stratification of the He/H abundance ratio in the nebula can be excluded. The central star had been analyzed by means of NLTE model atmospheres. From line profile fits we derive an effective temperature of T_eff=120±15kK and a surface gravity of logg=5.5±0.3. The photosphere is dominated by helium and hydrogen (n_H/n_He=0.5 by number). We find a solar nitrogen abundance (n_N/n_He=0.001). Neither carbon nor oxygen lines are exhibited in its spectrum which sets the upper limits to n_C/n_He<0.004 and n_O/n_He<0.008 which are the solar abundance ratios. These abundances indicate that the central star of LoTr4 can be a "born-again post-AGB star". This scenario has also been invoked for the evolution of PG1159 stars. In this picture the O(He) stars, LoTr4 and K1-27, might either be direct progenitors of the PG1159 stars or they have experienced less drastic mass-loss events.