  Many black hole X-ray transients are in a low state for several decades until
an outburst occurs. We interpret this outburst behaviour as a marginal
occurrence of a dwarf nova type disk instability in the cool outer accretion
disk. We compute the disk evolution including evaporation of matter from the
cool thin disk. This evaporation process causes the transition to a hot coronal
flow. The efficiency depends on the black hole mass.
  The new results are the dependence of the outburst recurrence time on the
mass transfer rate from the companion star and the fraction of the matter which
is accumulated in the disk for the outburst. We determine a lower limit of the
mass transfer rate, below which no disk instability can be triggered. We find
that for rates slightly lower than those in the known black hole X-ray
transients the disk would be stationary. We argue that many such optically
faint black hole X-ray binaries with stationary cool accretion disks exist.
