  We test the truncated disc models using multiwavelength (optical/UV/X-ray)
data from the 2005 hard state outburst of the black hole SWIFT J1753.5-0127.
This system is both fairly bright and has fairly low interstellar absorption,
so gives one of the best datasets to study the weak, cool disc emission in this
state. We fit these data using models of an X-ray illuminated disc to constrain
the inner disc radius throughout the outburst. Close to the peak, the observed
soft X-ray component is consistent with being produced by the inner disc, with
its intrinsic emission enhanced in temperature and luminosity by reprocessing
of hard X-ray illumination in an overlap region between the disc and corona.
This disc emission provides the seed photons for Compton scattering to produce
the hard X-ray spectrum, and these hard X-rays also illuminate the outer disc,
producing the optical emission by reprocessing.
  However, the situation is very different as the outburst declines. The
optical is probably cyclo-synchrotron radiation, self-generated by the flow,
rather than tracing the outer disc. Similarly, limits from reprocessing make it
unlikely that the soft X-rays are directly tracing the inner disc radius. This
is seen more clearly in a similarly dim low/hard state spectrum from XTE
J1118+480. The very small emitting area implied by the relatively high
temperature soft X-ray component is completely inconsistent with the much
larger, cooler, UV component which is well fit by a truncated disc. We
speculate on the origin of this component, but its existence as a clearly
separate spectral component from the truncated disc in XTE J1118+480 shows that
it does not simply trace the inner disc radius, so cannot constrain the
truncated disc models.
