λ4363/Hγ vs. λ5007/Hβ (top) and 2MASS NIR colour−colour (bottom) diagnostic diagrams of PNe and SySts. The triangles and squares correspond to PNe and SySts, respectively. The red and green pentagons represent the bipolar nebulae, M 2−9, Mz 3, and Hen 2−104, with MyCn 18 as a special case. The colours represent the different sources (see Tables A.1 and A.2) from which the data were obtained. The dashed box in the bottom left-hand corner of the optical diagnostic diagram defines a small region where the identification of the sources as SySts, Young PNe, or PNe is difficult due to the uncertainty in the weak [O iii] 4363 Å line, and the two diagonal lines correspond to an [O iii] 5007/4364 line ratio of 13.1 (upper line) and 27.4 (lower line). The red and green crosses in the 2MASS NIR diagram correspond to K and M giants respectively, and the black arrow corresponds to 3 mag extinction in the V band.