  INTEGRAL monitoring of the Galactic Plane is revealing a growing number of
recurrent X-ray transients, characterised by short outbursts with very fast
rise times (~ tens of minutes) and typical durations of a few hours. A
substantial fraction of these sources are associated with OB supergiants and
hence define a new class of massive X-ray binaries, which we call Supergiant
Fast X-ray Transients. Characterisation of the astrophysical parameters of
their counterparts is underway. So far, we have found a number of late O and
early B supergiants of different luminosities at a large range of distances.
Nothing in their optical properties sets them apart from classical Supergiant
X-ray Binaries. On the other hand, there is now rather concluding evidence that
persistent supergiant X-ray binaries also show fast outbursts. This suggests a
continuum of behaviours between typical persistent supergiant systems and
purely transient systems, but offers very little information about the physical
causes of the outbursts.
