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 b e a binary with a period of 1196 +/- 6 days. The photospheric abundanc es are deduced using high resolution, high signal-to-noise optical spe ctra and prove that AC Her is another RV Tauri star displaying depleti on of refractory elements, strengthening the relation between binarity and the depletion process. Our full ISO-SWS spectrum shows that the u nusual broad emission feature at 8-12 mu m is due to a high crystallis ation fraction of the circumstellar silicates. We review all observati onal evidence that the circumstellar material is trapped in a long-liv ed disc in the system similar to what is observed in the Red Rectangle nebula.