Third-dredge-up oxygen in planetary nebulae

The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf gal axy appear to result from nearly twin stars, except that third-dredge-up ca rbon is more abundant in He 2-436. A thorough photoionization-model analysi s implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2- 436, indicative of third-dredge-up oxygen enrichment. The enrichment of oxy gen with respect to carbon is (7+/-4)%. Excess nitrogen in Wray 16-423 sugg ests third dredge-up of late-CN-cycle products even in these low-mass, inte rmediate-metallicity stars.