  It is commonly assumed that high mass X-ray binary (HMXB) populations are
little-affected by metallicity. However, the massive stars making up their
progenitor systems depend on metallicity in a number of ways, not least through
their winds. We present simulations, well-matched to the observed sample of
Galactic HMXBs, which demonstrate that both the number and the mean period of
HMXB progenitors can vary with metallicity, with the number increasing by about
a factor of three between solar and SMC metallicity. However, the SMC
population itself cannot be explained simply by metallicity effects; it
requires both that the HMXBs observed therein primarily sample the older end of
the HMXB population, and that the star formation rate at the time of their
formation was very large.
