INTRODUCTION

RV Tau variables appear to be either blue-loop stars, at the very end of their evolution along the asymptotic giant branch (AGB), or are already post-AGB stars evolving bluewards across the instability strip (Jura 1986). Some years ago, Luck and Bond (1989) found evidence for systematic s-process under-abundances in field RV Tau variables, and stars like them. If this were true, it would have serious implications for our accepted view of AGB evolution, the nature of RV Tau variables, and the nucleosynthesis of s-process elements in the universe.

Luck and Bond (1989) noted that the s-process abundances were reminiscent of those found in extreme halo red giants. Yet RV Tau variables have much higher metallicities than extreme halo stars. Luck and Bond suggested that either:

1. the stars reflect their original s-process deficiencies relative to Fe, but severe mass-loss has reduced the hydrogen abundance in the atmosphere, thereby raising the Fe/H ratios to the observed levels.
2. the stars have normal s-process abundances, but because of their low 2nd ionization potentials, these elements are selectively over-ionized by Lyman-continuum photons. The key to this possibility is the observed systematic deficiency in Sc in these stars; the only light metal that shares a sufficiently low 2nd ionization potential.

These two alternatives could be tested by observing RV Tau variables, and stars like them, in globular clusters, and comparing them with their red giant companions. If a hydrogen deficiency caused the s-process deficiencies, RV Tau variables would show higher [Fe/H] ratios than the normal red giants, but the same [s-process/Fe] ratios. If over-ionization were the cause, RV Tau variables should show the same [Fe/H] ratios as the red giants, but lower [s-process/Fe] ratios.

In order to test these possibilities, I analysed the spectra of three RV Tau, or RV Tau-like variables, in globular clusters, and their red giant companions (Russell, 1997). My conclusion was that RV Tau variables in globular clusters show almost none of the characteristics of the stars in the field, as determined by Luck and Bond (1989), and references therein. The [Fe/H] is quite normal, and the s-process elements show no evidence for depletion relative to iron. If anything, s-process elements, and scandium, show enhancements compared with their red giant companions.

This situation was somewhat puzzling, and initially it was thought that the chemical characteristics of RV Tau variables as a class, were poorly determined due to the small numbers of objects studied so far. Fortunately, some degree of order and consensus have been arrived at through the recent work of Gonzalez, Lambert and colleagues (Gonzalez & Wallerstein, 1994; Gonzalez, Lambert, & Giridhar, 1997a; Gonzalez, Lambert, & Giridhar, 1997b; and Gonzalez & Lambert, 1997; Giridhar, Rao, & Lambert, 1994).

The intention of this paper, is to combine the results of the different chemical abundance studies of RV Tau variables, to see if we can throw any new light on the final stages in the evolution of stars similar to our own sun, and the nucleosynthetic processes that occur.

© Copyright Astronomical Society of Australia 1997