  HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5-8 V
+ O5-8 V) and an O8 supergiant. It is also a well known non-thermal radio
emitter. We observed the radio emission of HD 167971 with the Very Large Array
(VLA) and the Australia Telescope Compact Array (ATCA). By combining these data
with VLA archive observations we constructed a radio lightcurve covering a
20-yr time-range. We searched for, but failed to find, the 3.3-day
spectroscopic period of the binary in the radio data. This could be due to the
absence of intrinsic synchrotron radiation at the colliding-wind region between
the two components of the eclipsing binary, or due to the large amount of
free-free absorption that blocks the synchrotron radiation. We are able to
explain many of the observed characteristics of the radio data if the
non-thermal emission is produced in a colliding-wind region between the
supergiant and the combined winds of the binary. Furthermore, if the system is
gravitationally bound, the orbital motion occurs over a period of ~ 20 years,
or longer, as suggested by the long-term variability in the radio data. We
argue that the variability is due to the free-free absorption that changes with
orbital phase or may also in part be due to changes in separation, should the
orbit be eccentric.
