  I argue that it is very unlikely that X-rays from quiescent black-hole
low-mass X-ray binary transients are emitted by coronae of companion stars. I
show that in a simple model in which these X-rays are emitted by an ADAF
filling the inner part of an unsteady, dwarf-nova type disc, the X-ray
luminosity is correlated with the orbital period. I predict what values of
X-ray luminosities from black-hole transient systems should be observed by
Chandra and XMM-Newton.
