On the relative C,N,O abundances and the evolutionary status of yellow symbiotic stars.

Symbiotic binaries consist of a cool giant and a hot radiation source, which is ionizing the nebula associated with the system. Yellow symbiotic stars can be divided according to their IR colors into s-type (stellar) and d'-type (dust) systems. The s-types are relatively narrow systems, with orbital periods of several hundred days. They contain high density emission nebulae with N(e) about 10 exp 9/cu cm, ionized by a hot source of 75,000 K or greater. A large fraction of s-type systems belong to an old metal-poor stellar population. The d'-types are probably wider systems with very long binary periods. They show dust emission, a lower density nebula of N(e) about 10 exp 7/cu cm, and ionizing sources of about 60,000 K. We determine relative C,N,O abundances from strong UV nebular emission lines. Of 10 investigated systems, five show C,N,O abundances typical of cool giants after CN cycle burning and a first dredge-up phase. Four yellow systems are enriched in carbon beyond expectation for normal G, K, or M giants. These systems may be closely related to Ba stars or early R-type carbon stars, which also show overabundances in carbon. This suggests that in some yellow symbiotics mass transfer may be responsible for abundance modifications. As a by-product we reclassify AS 210 as a symbiotic carbon Mira.