2002A&A...396..877K

We present mid-infrared spectrophotometric results obtained with the ISO on the peculiar X-ray binary Cygnus X-3 in quiescence, at orbital phases 0.83 to 1.04. The 2.4-12µm continuum radiation observed with ISOPHOT-S can be explained by thermal free-free emission in an expanding wind with, above 6.5µm, a possible additional black-body component with temperature T∼250K and radius R∼5000R_☉ at 10kpc, likely due to thermal emission by circumstellar dust. The observed brightness and continuum spectrum closely match that of the Wolf-Rayet star WR 147, a WN8+B0.5 binary system, when rescaled at the same 10 kpc distance as Cygnus X-3. A rough mass loss estimate assuming a WN wind gives ∼1.2x10^–4M_☉/yr. A line at ∼4.3µm with a more than 4.3σ detection level, and with a dereddened flux of 126mJy, is interpreted as the expected He I 3p-3s line at 4.295µm, a prominent line in the WR147 spectrum. These results are consistent with a Wolf-Rayet-like companion to the compact object in Cyg X-3 of WN8 type, a later type than suggested by earlier works.