  Neutron star (NS) masses and radii can be estimated from observations of
photospheric radius-expansion X-ray bursts, provided the chemical composition
of the photosphere, the spectral colour-correction factors in the observed
luminosity range, and the emission area during the bursts are known. By
analysing 246 X-ray bursts observed by the Rossi X-ray Timing Explorer from 11
low-mass X-ray binaries, we find a dependence between the persistent spectral
properties and the time evolution of the black body normalisation during the
bursts. All NS atmosphere models predict that the colour-correction factor
decreases in the early cooling phase when the luminosity first drops below the
limiting Eddington value, leading to a characteristic pattern of variability in
the measured blackbody normalisation. However, the model predictions agree with
the observations for most bursts occurring in hard, low-luminosity, 'island'
spectral states, but rarely during soft, high-luminosity, 'banana' states. The
observed behaviour may be attributed to the accretion flow, which influences
cooling of the NS preferentially during the soft state bursts. This result
implies that only the bursts occurring in the hard, low-luminosity spectral
states can be reliably used for NS mass and radius determination.
