  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. We probed
the innermost region of the circumstellar dust shell of the deeply embedded WR
star WR 118. 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. 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+-1.1 mas.
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 ~60
days (for d=3 kpc), making WR 118 the shortest-period pinwheel nebula known so
far.
