SIMBAD references

We present accurate radial velocity measurements on the pulsating extremely iron-deficient post-AGB object HD 52961 and the RV Tauri star EN TrA (HD 131356) proving them to be binaries. Our long-term photometric monitoring campaign shows that the RV Tauri photometric class ``b'' phenomenon in HD 52961 is due to variable circumstellar extinction during orbital motion. By comparing carefully the observational characteristics of RV Tauri stars and the class of extremely iron-deficient post-AGB objects we conclude that binarity is a widespread phenomenon in the RV Tauri class of objects. The observed chemical depletion patterns, weak circumstellar CO emission, peculiar spectral energy distribution and the difference in photospheric class of the RV Tauri objects can all be naturally explained by assuming that the circumstellar material is not freely expanding, but trapped in the binary system.