Strong dust processing in circumstellar discs around 6 RV Tauri stars - Are dusty RV Tauri stars all binaries?S. De Ruyter, H. Van Winckel, C. Dominik, L. B. F. M Waters and H. DejongheA&A, 435 1 (2005) 161-166DOI: https://doi.org/10.1051/0004-6361:20041989