It has been well established that RV Tauri variables possess infrared emission far in excess of their expected blackbody continuum, arising from their extended cool dust envelopes (Gehrz & Woolf 1970; Gehrz 1972; Gehrz & Ney 1972). Recently, Lloyd Evans (1985) and Goldsmith et al. (1987) have given detailed descriptions of the near-infrared properties of RV Tauri stars. In this paper we present an analysis of the IRAS data of RV Tauri stars with the help of the far-infrared two-colour diagram and a grid computed using a simple model of the dust envelope. Such two-colour plots have already been employed extensively by several investigators to study the circumstellar envelopes around oxygen-rich and carbon-rich objects which are in the late stages of stellar evolution (Hacking et al. 1985; Zuckerman & Dyck 1986; van der Veen & Habing 1988; Willems & de Jong 1988).
Table 1 summarizes the basic data on the 17 objects detected at 60 μm. Apart from the IRAS identification and the flux densities at 12-, 25-, 60- and 100-μm wavebands, it gives the spectroscopic groups of Preston et al. (1963), the light-curve classes of Kukarkin et al. (1969) and the periods of light variation. The list, which contains about 20 per cent of all the known RV Tauri stars, is essentially the same as that given by Jura (1986). The spectroscopic subgroups are from either Preston et al. (1963) or Lloyd Evans (1985). 1 #& #& #& #& #& #& # & #& #& # Name&&4Flux density (Jy)a Variable&IRAS&12 μm&25 μm &60 μm&100 μm&Sp.&Period&Light-& T0(K) &&&&&&group&(d)&curve &&&&&&&&type TW Cam&04166+5719&8.27&5.62&1.82&<1.73&A&85.6&a&555 RV Tau&04440+2605&22.53&18.08&6.40&2.52&A&78.9&b&460 DY Ori&06034+1354&12.44&14.93&4.12&<11.22&B&60.3&&295 CT Ori&06072+0953&6.16&5.57&1.22&<1.54&B&135.6&&330 SU Gem&06108+2734&7.90&5.69&2.16&<11.66&A&50.1&b&575 UY CMa&06160-1701&3.51&2.48&0.57&<1.00&B&113.9&a&420 U Mon&07284-0940&124.30&88.43&26.28&9.24&A&92.3&b&480 AR Pup&08011-3627&131.33&94.32&25.81&11.65&B&75.0&b&450 IW Car&09256-6324&101/06&96.24&34.19&13.07&B&67.5&b&395 GK Car&11118-5726&2.87&2.48&0.78&<12.13&B&55.6&&405 RU Cen&12067-4508&5.36&11.02&5.57&2.01&B&64.7&&255 SX Cen&12185-4856&5.95&3.62&1.09&<1.50&B&32.9&b&590 AI Sco&17530-3348&17.68&11.46&2.88&<45.62&A&71.0&b&480 AC Her&18281+2149&41.47&65.33&21.12&7.79&B&75.5&a&260 R Sct&18448-0545&20.88&9.30&8.10&<138.78&A&140.2&a R Sge&20117+1634&10.63&7.57&2.10&<1.66&A&70.6&b&455 V Vul&20343+2625&12.39&5.72&1.29&<6.96&A&75.7&a&690 aObserved by IRAS.