We propose to observe a selection of RV Tauri stars with the
 SWS. We will address the following questions: (i) what is the
 nature of the cool outer (AGB) shell of RV Tauri stars? (ii) what
 is the origin of the peculiar 'silicate' feature seen in many
 RV Tauri stars? (iii) what is the distribution of dust around
 RV Tauri stars? (iv) is there evidence for iron oxide grains in
 the envelopes of RV Tauri stars, which may have originated in the
 AGB progenitor? SWS observations of RV Tauri stars will provide
 answers to such questions and will lead to important new information
 on the evolutionary history of these poorly-understood objects.