The nitrogen abundance in NGC 55

The purpose of this paper is to propose an interpretation for the line spectrum of NGC 55, whose most remarkable feature is the absence of the generally noticeable 6584Nii]. From the physical conditions of the ionized gas responsible of the observed emission, we infer that the N(N)/N(O) abundance ratio is significantly smaller than usual, probably reflecting a nitrogen underabundance in a medium of temperatures 9100 K and densities 500 cm^–3.