We present results of NTT spectroscopy of the one known planetary nebula ( PN ) in the dwarf spheroidal galaxy Fornax , a gas-deficient Local Group galaxy that stopped its star formation activity a few hundred million years ago .
We detected the [ O iii ] \lambda 4363 line with a signal-to-noise ratio of \sim 22 .
For the first time we detected the weak [ S ii ] \lambda \lambda 6717,6731 lines ( I ( 6717+6731 ) \approx 0.01 I ( H \beta ) ) , determined the electron number density ( N _ { e } ( SII ) = 750 cm ^ { -3 } ) , and calculated O , N , Ne , Ar , S , Cl , Fe , He and C abundances .
The abundance analysis presented here is based on the direct calculation of the electron temperature T _ { e } and yields an oxygen abundance of 12+log ( O/H ) = 8.28 \pm 0.02 .
The analysis of the O , Ne , Ar and S abundances shows that the original ISM oxygen abundance was 0.27 \pm 0.10 dex lower and that third-dredge-up self-pollution in oxygen took place .
The blue spectrum shows weak Wolf-Rayet features , and the progenitor star is classified as a weak emission-line star .
Four of the five PNe in dwarf spheroidal galaxies are now known to show WR wind features .
Overall , the metallicity of the progenitor of the PN fits in well with stellar spectroscopic abundances derived in previous studies as well as with the stellar age-metallicity relation of Fornax .