On the interpretation of the extinction curves of R Coronae Borealis stars

The available extinction curves for some RCB stars (R CrB, RY Sgr, MV Sgr and V348 Sgr) have been analysed, making use of the method of regularization for solution of ill-posed problems. This approach allows an estimate of dust grain sizes responsible for fadings of RCB stars. A model involving graphite grains is shown to be preferable for the explanation of practically all available extinction curves. This conclusion is supported by estimates of the equilibrium temperatures of both 'old' and newly formed dust. The size distributions of dust grains have a distinct peak-like form with typical sizes 200-700 Angstrom and widths 20-60 Angstrom. This is consistent with dust forming in separate clouds and the dust grains within each cloud having similar sizes.