The nature of RV Tauri variables. I - Spectrophotometric atlas of selected field stars

Multiphase observations of a selected sample of field RV Tauri variables are presented, consisting of 10-A resolution spectrophotometry in the 3500-6200-A wavelength range. The data confirm the previous suggestion that most of the RV Tau variables exhibit clear deviations from normal supergiants which cannot be related simply to metallicity. Evidence is found for strong molecular absorption (CN for AC Her and EP Lyr; TiO for R Sct and V Vul) at phases where the inferred stellar temperature is inconsistent with such absorption, possibly due to atmospheric shocks which arise in at least two separate episodes (most notably, in the rise from primary and secondary minimum).