The reddening law for circumstellar dust can in principle be determined if it is assumed that the energy radiated at infrared wavelengths by the dust exactly balances that absorbed from the star. The method is attempted for a number of early-type stars, and no evidence is found that the ratio of total to selective absorption, AV/EB–V, differs from that of interstellar reddening. An apparent reduction of AV/Ev–1 relative to the interstellar case is probably explained by free–free emission from the circumstellar gas.
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D. A. Allen (1976) studied this question.