Context : Most Wolf-Rayet stars ( WR ) of WC9 sub-type exhibit a dusty circumstellar envelope , but it is still a matter of debate how dust can form in their harsh environment .
In a few cases , a pinwheel-like structure of the dusty envelope has been detected .
Therefore , it has been suggested that dust formation in all dusty WR stars might be linked to colliding winds in a binary system .

Aims : We probed the innermost region of the circumstellar dust shell of the deeply embedded WR star WR 118 .

Methods : We carried out spectro-interferometric observations using the AMBER instrument of ESO ’ s Very Large Telescope Interferometer in low-spectral resolution mode ( R = 35 ) .
The K -band observations were obtained with three 1.8 m telescopes spanning projected baselines between 9.2 and 40.1 m .

Results : At high spatial frequencies , the AMBER visibilities exhibit a prominent lobe , indicating that the envelope contains one or several zones with a large local intensity gradient .
The strong closure phase signal clearly shows that the circumstellar envelope of WR 118 can only be described by an asymmetric intensity distribution .
We show that a pinwheel nebula seen at low inclination is consistent with the AMBER data .
Its size was determined to be 13.9 \pm 1.1 mas .

Conclusions : WR 118 possibly harbors a pinwheel nebula , which suggests a binary nature of the system .
According to our best model , the period of the system would be \approx 60 days ( for d = 3 kpc ) , making WR 118 the shortest-period pinwheel nebula known so far .