We have calculated spectral energy distributions and synthetic IRAS colours of carbon-rich long-period variables. On top of dynamical models for the stellar atmosphere and circumstellar envelope which consistently treat the time-dependent hydrodynamics, the formation of amorphous carbon grains and grey radiative transfer, frequency-dependent radiative transfer calculations have been carried out in the range between 1 micrometer and 290 micrometer. Calculating the IRAS colours we find that the models lie in regions I (stars without circumstellar shells) and VII (variable stars with carbon-rich circumstellar shells) of the IRAS two colour diagram in accordance with observational results for carbon stars. They form an almost linear sequence (near the black body line) reflecting the different mass loss rates. We compare our results to empirical formulae which link the mass loss rate to the observed flux at 60 micrometer. Furthermore, we have also investigated qualitatively the effects of dust with alpha-SiC grains and of detached shells. The SiC-contribution causes mainly a blue-shift of about 0.2 mag in the [12]-[25] colour. Adding a detached shell results in higher [25]-[60] values partly shifting the models into region VIa (non-variable stars with relatively cold dust at large distances) of the IRAS two colour diagram as expected from observations.
A&A 324, 617