Astron. Astrophys. 336, L17-L20 (1998)

1. Introduction

The RV Tauri class of objects is characterised by photometric variations showing alternating deep and shallow minima, that are interpreted as stellar pulsations. RV Tauri stars have high luminosity and often show a large IR excess due to thermal emission from circumstellar dust grains (Jura 1986). These properties indicate that RV Tauri stars are post-asymptotic giant branch (post-AGB) objects. In a recent series of papers (Giridhar et al. 1994, 1998; Gonzalez et al. 1997a,b) is was shown that the photospheric abundances of most field RV Tauri stars are characterized by depletion of refractory elements by dust formation. The difference in metal content, labelled `A' (strong-lined objects), `B' (weak-lined objects with enhanced CN and CH) by Preston et al. (1963), in fact reflects a difference in efficiency of the depletion process and not in initial metallicity. The true metal deficient RV Tauri stars, not affected by depletion seem to be the `C' class objects (weak-lined objects with absent or weak CN and CH) which include the globular cluster RV Tauri variables (Giridhar et al., 1998).

The depletion pattern of RV Tauri stars is similar to (but less extreme than) that of a small group of binary post-AGB stars, among which the RV Tauri star HD 52961 (Waelkens et al. 1991a; Van Winckel et al. 1992) and the Red Rectangle and its central star HD 44179 (Van Winckel et al. 1995 and references therein). The binarity of these objects and the observed presence of dust discs around some of them (e.g. Waelkens et al. 1991b; Roddier et al. 1995), and the similar patterns observed in Boötis stars (Venn & Lambert 1991) led Waters et al. (1992) to suggest that the gas-dust separation needed to account for the depletion pattern is likely to take place in a (long-lived) disc.

In this letter we demonstrate that one of the proto-types of the RV Tauri class, AC Her, is in fact member of a wide binary system, and that it shows a pronounced depletion pattern of refractory elements in its photosphere. The previously reported C-rich nature of the star (based on the C/Fe ratio), is entirely due to this depletion. In addition, on the basis of new ISO observations, we show that the circumstellar dust is predominantly oxygen-rich and shows strong crystalline silicates. The dust properties of AC Her closely resemble those of the dust in the long-live disc in the Red Rectangle.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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