  Classical Supergiant X-ray Binaries (SGXBs) and Supergiant Fast X-ray
Transients (SFXTs) are two types of High-mass X-ray Binaries (HMXBs) that
present similar donors but, at the same time, show very different behavior in
the X-rays. The reason for this dichotomy of wind-fed HMXBs is still a matter
of debate. Among the several explanations that have been proposed, some of them
invoke specific stellar wind properties of the donor stars. Only dedicated
empiric analysis of the donors' stellar wind can provide the required
information to accomplish an adequate test of these theories. However, such
analyses are scarce. To close this gap, we perform a comparative analysis of
the optical companion in two important systems: IGR J17544-2619 (SFXT) and Vela
X-1 (SGXB). We analyse the spectra of each star in detail and derive their
stellar and wind properties. We compare the wind parameters, giving us an
excellent chance of recognizing key differences between donor winds in SFXTs
and SGXBs. We find that the stellar parameters derived from the analysis
generally agree well with the spectral types of the two donors: O9I (IGR
J17544-2619) and B0.5Iae (Vela X-1). An important difference between the
stellar winds of the two stars is their terminal velocities v_inf=1500km/s in
IGR J17544-2619 and v_inf=700km/s in Vela~X-1, which has important consequences
on the X-ray luminosity of these sources. Their specific combination of wind
speed and pulsar spin favours an accretion regime with a persistently high
luminosity in Vela X-1, while it favours an inhibiting accretion mechanism in
IGR~J17544-2619. Our study demonstrates that the wind relative velocity is
critical in the determination of the class of HMXBs hosting a supergiant donor,
given that it may shift the accretion mechanism from direct accretion to
propeller regimes when combined with other parameters.
