  High-resolution IRAS maps are used to search for the presence of stellar-wind
bow-shocks around high-mass X-ray binaries (HMXBs). Their high space
velocities, recently confirmed with Hipparcos observations, combined with their
strong stellar winds should result in the formation of wind bow-shocks. Except
for the already known bow-shock around Vela X-1 (Kaper et al. 1997), we do not
find convincing evidence for a bow-shock around any of the other HMXBs. Also in
the case of (supposedly single) OB-runaway stars, only a minority appears to be
associated with a bow-shock (Van Buren et al. 1995). We investigate why wind
bow-shocks are not detected for the majority of these OB-runaway systems: is
this due to the IRAS sensitivity, the system's space velocity, the stellar-wind
properties, or the height above the galactic plane? It turns out that none of
these suggested causes can explain the low detection rate (~40%). We propose
that the conditions of the interstellar medium mainly determine whether a wind
bow-shock is formed or not. In hot, tenuous media (like inside galactic
superbubbles) the sound speed is high (~100 km/s), such that many runaways move
at subsonic velocity through a low-density medium, thus preventing the
formation of an observable bow-shock. Superbubbles are expected (and observed)
around OB associations, where the OB-runaway stars were once born. Turning the
argument around, we use the absence (or presence) of wind bow-shocks around OB
runaways to probe the physical conditions of the interstellar medium in the
solar neighbourhood.
