  We present spectroscopy of the P~Cygni profile of the 1.083-micron He I line
in the WC7 + O5 colliding-wind binary (CWB) WR 140 (HD 193793), observed in
2008, before its periastron passage in 2009, and in 2016-17, spanning the
subsequent periastron passage. Both absorption and emission components showed
strong variations. The variation of the absorption component as the O5 star was
occulted by the wind-collision region (WCR) sets a tight constraint on its
geometry. While the sightline to the O5 star traversed the WCR, the strength
and breadth of the absorption component varied significantly on time-scales of
days. An emission sub-peak was observed on all our profiles. The variation of
its radial velocity with orbital phase was shown to be consistent with
formation in the WCR as it swung round the stars in their orbit. Modelling the
profile gave a measure of the extent of the sub-peak forming region. In the
phase range 0.93-0.99, the flux in the sub-peak increased steadily,
approximately inversely proportionally to the stellar separation, indicating
that the shocked gas in the WCR where the line was formed was adiabatic. After
periastron, the sub-peak flux was anomalously strong and varied rapidly,
suggesting formation in clumps down-stream in the WCR. For most of the time,
its flux exceeded the 2-10-keV X-ray emission, showing it to be a significant
coolant of the shocked wind.
