  We attempt to determine the driver for clumping in hot-star winds by
extending the measure of the spectral variability level of Galactic Wolf-Rayet
stars to by far the hottest known among them, the WN2 star WR 2 and the WO2
stars WR 102 and WR 142. These three stars have T* = 140 kK and 200 kK, the
last two being well above the bulk of WR stars with T* ~ 40-120 kK. This full
temperature range for WR stars is much broader than that of their O-star
progenitors (~30-50 kK), so is better suited to look for any temperature
dependence of wind clumping. We have obtained multiple observations with high
signal-to-noise, moderate-resolution spectroscopy in search of smallscale
variability in the strong emission lines from the dense winds of these three
extreme stars, and find a very low-level of variability in both stars.
Temperature and terminal velocity are correlated, so faster winds show a lower
variability, though this trend goes against any predictions made involving Line
Deshadowing Instability (LDI) only, implying that instabilities intrinsic to
LDI are not the main source of wind clumping. Instead, it could be taken as
support for the suggestion that clumps are caused by a sub-surface convection
zone (SSCZ) at T ~ 170 kK, since such an SSCZ would have little opportunity to
operate under the hydrostatic surface of these hottest WR stars. It is still
possible, however, that an SSCZ-related driver could interact with nonlinear
line instability effects to enhance or possibly even produce clumps.
