  We present results on spectral variability of two Galactic black hole X-ray
binaries, GRO J1655-40 and GX 339-4, in the hard state. We confirm a transition
in behaviour of the photon index with luminosity, such that the well known
decrease in X-ray photon index with decreasing luminosity only continues down
to L_bol ~ 0.01 L_Edd . Below this point the photon index increases again. For
Comptonisation models, this implies that the ratio of the Compton luminosity to
seed photon luminosity, lh/ls, changes with bolometric luminosity, consistent
with a scenario where seed photons change from cyclo-synchrotron at the lowest
luminosities to those from a truncated disc. Alternatively, the transition
could mark the point below which the non-thermal jet starts to dominate, or
where reprocessed photons replace the viscous ones in an outflowing corona
model.
