The recent finding by Chevalier & Ilovaisky (1998) that OB-supergiant X-ray
binaries have relatively large runaway velocities whereas Be/X-ray binaries
have low runaway velocities, provides confirmation of the current models for
the formation of these two types of systems. These predict a difference in
runaway velocity of an order of magnitude. This difference basically results
from the variation of the fractional helium core mass as a function of stellar
mass, in combination with the conservation of orbital angular momentum during
the mass transfer phase that preceded the formation of the compact object in
the system. This combination results into: (i) Systematically narrower
pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and
(ii) A larger fractional amount of mass ejected in the supernovae in high-mass
systems relative to systems of lower mass. Regardless of possible kick
velocities imparted to neutron stars at birth, this combination leads to a
considerable difference in average runaway velocity between these two groups.
The observed low runaway velocities of the Be/X-ray binaries confirm that in
most cases not more than 1 to 2Msun was ejected in the supernovae that produced
their neutron stars. This, in combination with the --on average-- large orbital
eccentricities of these systems, indicates that their neutron stars must have
received a velocity kick in the range 60 - 250 km/s at birth.Comment: reduced abstract, 13 pages, accepted by A&