  Stellar-mass black hole binary systems in the luminous X-ray states show a
strong quasi-periodic oscillation (QPO) in their Comptonised emission. The
frequency of this feature correlates with the ratio of a disc to Comptonised
emission rather than with total luminosity. Hence it changes dramatically
during spectral transitions between the hard and soft states. Its amplitude is
also strongest in these intermediate states, making them an important test of
QPO models. However, these have complex spectra which generally require a disc
and two separate Comptonisation components, making it difficult to uniquely
derive the spectral parameters. We build a new energy-conserving model of the
accretion flow, SSsed model, which assumes a fixed radial emissivity but with a
changing emission mechanism. This is similar to the agnsed model in xspec but
tuned to be more suitable for stellar mass black holes. It uses a combination
of the disc luminosity and temperature to constrain the inner radius of the
(colour temperature corrected) blackbody disc, separating this from the more
complex Comptonisation spectra emitted inwards of this radius. We show a pilot
study of this model fit to hundreds of RXTE spectra of the black hole binary
XTE J1550-564. We show that the derived disc radius tightly anti-correlates
with the central frequencies of the low-frequency QPO detected in the same
observations. The relation is consistent with the quantitative predictions of
Lense-Thirring precession of the entire inner Comptonisation regions for the
assumed system parameters. This supports the scenario that low-frequency QPOs
are caused by Lense-Thirring precession.
