Fig. 8. ) vs. He/H diagram for the 15 bipolar PNe (dots with errorbars). The three helium-rich nebulae on the top of the diagram are, from right to left, M 3-2, He 2-111, and NGC 6537. Triangles are data for the bipolar PN M 1-75 (Guerrero et al. 1995), MyCn 18, M 1-8, and M 3-3 (KB94). The region at the top-right of the dashed lines is the locus of type I PNe (Peimbert & Torres-Peimbert 1983).