Conclusions

In the [12]–[25], [25]–[60] colour diagram, RV Tauri stars populate cooler temperature regions (T < 600 K), distinctly different from those occupied by the oxygen and carbon Miras. Using a simple model in which

  1. the envelope is spherically symmetric,
  2. the IR-emitting grains are predominantly of the same kind, and
  3. in the infrared the absorption efficiency Qabs(ν)∝ν,
we find that the IRAS fluxes are consistent with the density in the envelope ρ(r)∝r-2, where r is the radial distance. Such a dependence for the dust density implies that the mass-loss rates in RV Tauri stars have not reduced considerably during the recent past, contrary to the suggestion by Jura (1986). In the two-colour diagram, the blackbody line and the line corresponding to ρ(r)∝r-2.2 nearly overlap and the present data are insufficient to resolve between the two cases. The latter case is more physically reasonable, however.

The spectroscopic subgroups A and B are well separated in the IRAS two-colour diagram, with group B objects having systematically cooler dust envelopes. If we consider only the objects detected by IRAS, we find that stars belonging to group B show systematically larger excess at L band for a given excess at K. Apparently, there is no correlation between the light-curve types (RVa and RVb) and the far-infrared behaviour of these objects. It is fairly certain that the physical properties, including the chemical composition, of the embedded stars are directly reflected by those of the dust grains. Most probably, the grain formation process in RV Tauri stars is continuous and not sporadic as suggested by Goldsmith et al. (1987).