  We present a radio lightcurve of the O5.5 III(f+) star HD 168112, based on
archive data from the Very Large Array (VLA) and the Australia Telescope
Compact Array (ATCA). The fluxes show considerable variability and a negative
spectral index, thereby confirming that HD 168112 is a non-thermal radio
emitter. The non-thermal radio emission is believed to be due to synchrotron
radiation from relativistic electrons that have been Fermi accelerated in
shocks. For HD 168112, it is not known whether these shocks are due to a
wind-wind collision in a binary system or to the intrinsic instability of the
stellar wind driving mechanism. Assuming HD 168112 to be a single star, our
synchrotron model shows that the velocity jump of the shocks should be very
high, or there should be a very large number of shocks in the wind. Neither of
these is compatible with time-dependent hydrodynamical calculations of O star
winds. If, on the other hand, we assume that HD 168112 is a binary, the high
velocity jump is easily explained by ascribing it to the wind-wind collision.
By further assuming the star to be an eccentric binary, we can explain the
observed radio variability by the colliding-wind region moving in and out of
the region where free-free absorption is important. The radio data presented
here show that the binary has a period between one and two years. By combining
the radio data with X-ray data, we find that the most likely period is about
1.4 yr.
