ABUNDANCE ANALYSES OF TYPE-II CEPHEIDS IN GLOBULAR-CLUSTERS

We have determined the photospheric compositions of five luminous F-G variables in globular clusters: V11 in M2, V42 in M5, V84 in M5, V2 in M10, and V17 in M28, three of which are classified as RV Tau variable stars. Unlike the recent findings that the photospheres of field RV T au variables have been altered by a gas-dust separation process, none of these stars, except possibly V2 in M10, show such signs. We suggest that either a smaller carbon abundance in the photosphere of a globul ar cluster RV Tau star (compared to its field analogues) might be a fa ctor in the gas-dust separation process due to the lack of sufficient carbon grain formation, or the stellar winds and their efficacy as dus t-grain separators is influenced by the metallicity of the stellar env elope which, unlike the photosphere, is never subject to alteration by a dust-gas separation. Our analysis is most complete for V84, for whi ch we have estimated the abundances of 19 chemical elements. Compared to the other giants in M5, this star displays a very large excess of N a and a deficit of O. This finding is consistent with a recently propo sed mechanism of Na production in the ON-cycling region of metal-poor RGB stars. The value of [Al/Fe] for V84 is also high (also consistent with ON-cycling), but a comparison with the other cluster giants canno t be performed until Al is measured in their spectra. The s-process el ements are not significantly enhanced in the atmosphere of V84 relativ e to the other cluster members. (C) 1997 American Astronomical Society .