2003A&A...400..511M

Spectroscopic observations of 48 emission-line objects of M 33 have been obtained with the multi-object, wide field, fibre spectrograph AF2/WYFFOS at the 4.2 m WHT telescope (La Palma, Spain). Line intensities and logarithmic extinction, c_β, are presented for 42 objects. Their location in the Sabbadin & D'Odorico (1976A&A....49..119S) diagnostic diagram (Hα/[S II] vs. Hα/[N II]) suggests that >70% of the candidates are Planetary Nebulae (PNe). Chemical abundances and nebular physical parameters have been derived for the three of the six PNe where the 4363Å [O III] emission line was measurable. These are disc PNe, located within a galactocentric distance of 4.1kpc, and, to date, they are the farthest PNe with a direct chemical abundance determination. No discrepancy in the helium, Oxygen and Argon abundances has been found in comparison with corresponding abundances of PNe in our Galaxy. Only a lower limit to the sulphur abundance has been obtained since we could not detect any [S III] line. N/H appears to be lower than the Galactic value; some possible explanations for this under-abundance are discussed.