  Radio surveys of early-type stars have revealed a number of non-thermal
emitters. Most of these have been shown to be binaries, where the collision
between the two stellar winds is responsible for the non-thermal emission. HD
168112 is a non-thermal radio emitter, whose binary nature has only recently
been confirmed spectroscopically. We obtained independent spectroscopic
observations to determine its orbit, in addition to radio observations to see
if the thermal or non-thermal nature of the emission changes during the
periastron passage. We monitored HD 168112 spectroscopically for a 13 year time
span. From these data, we determined the orbital parameters, which we compared
to the previous results in the literature. From the spectral index of the radio
observations, we found how the nature of the emission changes as the system
goes through periastron. Combining our results with other literature data
allowed us to further constrain the orbital and stellar parameters. We find HD
168112 to have an orbital period of P = 512.17+0.41-0.11 d, an eccentricity of
e = 0.7533+0.0053-0.0124, and a mass ratio close to one. From our spectroscopic
modelling, we derived the stellar parameters, but we had difficulty arriving at
a spectroscopic mass ratio of one. The radio observations around periastron
show only thermal emission, suggesting that most of the synchrotron photons are
absorbed in the two stellar winds at that phase. Combining our data with the
optical interferometry detection, we could constrain the inclination angle to i
~ 63 deg, and the mass of each component to ~ 26 Msun. We have provided an
independent spectroscopic confirmation of the binary nature of HD 168112.
Although detected as a non-thermal radio emitter, near periastron the radio
emission of this highly eccentric system is thermal and is mainly formed in the
colliding-wind region. [abridged]
