  In this work the thermal emission over cm to sub-mm wavelengths from the
winds in short-period O+O-star binaries is investigated (potential non-thermal
emission is presently ignored). The calculations are based on three-dimensional
hydrodynamical models which incorporate gravity, the driving of the winds,
orbital motion of the stars, and radiative cooling of the shocked plasma. The
thermal emission arises from the stellar winds and the region where they
collide. We investigate the flux and spectrum from a variety of models as a
function of orbital phase and orientation of the observer, and compare to the
single star case. The emission from the wind-wind collision region (WCR) is
strongly dependent on its density and temperature, being optically thick in
radiative systems, and optically thin in adiabatic systems. The flux from
systems where the WCR is highly radiative, as investigated for the first time
in this work, can be over an order of magnitude greater than the combined flux
from identically typed single stars. This excess occurs over a broad range of
wavelengths from cm to sub-mm. In contrast, when the WCR is largely adiabatic,
a significant excess in the thermal flux occurs only below 100 GHz. Eccentric
systems may show order of magnitude or greater flux variability, especially if
the plasma in the WCR transitions from an adiabatic to a radiative state and
vice-versa - in such cases the flux can display significant hysteresis with
stellar separation. We further demonstrate that clumping can affect the
variability of radio lightcurves. We investigate the spectral index of the
emission, and often find indices steeper than +0.6. Our predictions are of
interest to future observations with the next generation of radio and sub-mm
telescopes (abridged).
