  Stellar winds of massive stars are known to be driven by line absorption of
UV photons, a mechanism which is prone to instabilities, causing the wind to be
clumpy. The clumpy structure hampers wind mass-loss estimates, limiting our
understanding of massive star evolution. The wind structure also impacts
accretion in high-mass X-ray binary (HMXB) systems. We analyse the
wavelength-dependent variability of X-ray absorption in the wind to study its
structure. Such an approach is possible in HMXBs, where the compact object
serves as an X-ray backlight. We probe different parts of the wind by analysing
data taken at superior and inferior conjunction. We apply excess variance
spectroscopy to study the wavelength-dependent soft X-ray variability of the
HMXB Cygnus X-1 in the low/hard spectral state. Excess variance spectroscopy
quantifies the variability of an object above the statistical noise as a
function of wavelength, which allows us to study the variability of individual
spectral lines. As one of the first studies, we apply this technique to
high-resolution gratings spectra provided by Chandra, accounting for various
systematic effects. The frequency dependence is investigated by changing the
time binning. The strong orbital phase dependence we observe in the excess
variance is consistent with column density variations predicted by a simple
model for a clumpy wind. We identify spikes of increased variability with
spectral features found by previous spectroscopic analyses of the same data
set, most notably from silicon in over-dense clumps in the wind. In the silicon
line region, the variability power is redistributed towards lower frequencies,
hinting at increased line variability in large clumps. In prospect of the
microcalorimetry missions that are scheduled to launch within the next decade,
excess variance spectra present a promising approach to constrain the wind
structure.
