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 of between one and two years .
By combining the radio data with X-ray data , we find that the most likely period is \sim 1.4 yr .