The binary RV Tauri star AC Her and evidence for a long-lived dust-disc

We present in this letter a homogeneous set of CORAVEL radial velocity measurements of the well studied RV Tauri star AC Her showing it to be a binary with a period of 1196 ± 6 days. The photospheric abundances are deduced using high resolution, high signal-to-noise optical spectra and prove that AC Her is another RV Tauri star displaying depletion of refractory elements, strengthening the relation between binarity and the depletion process. Our full ISO-SWS spectrum shows that the unusual broad emission feature at 8-12 μm is due to a high crystallisation fraction of the circumstellar silicates. We review all observational evidence that the circumstellar material is trapped in a long-lived disc in the system similar to what is observed in the Red Rectangle nebula