  We study the effects of the mutual interaction of hot plasma and cold medium
in black hole binaries in their hard spectral state on the value of the
truncation radii of accretion discs. We consider a number of different
geometries. In contrast to previous theoretical studies, we use a modern
energy-conserving code for reflection and reprocessing from cold media. We show
that a static corona above a disc extending to the innermost stable circular
orbit produces spectra not compatible with those observed. They are either too
soft or require a much higher disc ionization than that observed. This
conclusion confirms a number of previous findings, but disproves a recent study
claiming an agreement of that model with observations. We show that the cold
disc has to be truncated in order to agree with the observed spectral hardness.
However, a cold disc truncated at a large radius and replaced by a hot flow
produces spectra which are too hard if the only source of seed photons for
Comptonization is the accretion disc. Our favourable geometry is a truncated
disc coexisting with a hot plasma either overlapping with the disc or
containing some cold matter within it, also including seed photons arising from
cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal
and non-thermal parts.
