  Spectral measurements of thermonuclear (type-I) X-ray bursts from low mass
X-ray binaries have been used to measure neutron star (NS) masses and radii. A
number of systematic issues affect such measurements and have raised concerns
as to the robustness of the methods. We present analysis of the X-ray emission
from bursts observed from 4U 1608-52 at various persistent fluxes. We find a
strong dependence of the burst properties on the flux and spectral hardness of
the persistent emission before burst. Bursts occurring during the low-accretion
rate (hard) state exhibit evolution of the black body normalisation consistent
with the theoretical predictions of NS atmosphere models. However, bursts
occurring during the high-accretion rate (soft) state show roughly constant
normalisation, which is inconsistent with the NS atmosphere models and
therefore these bursts cannot be easily used to determine NS parameters. We
analyse the hard-state burst to put the lower limit on the neutron star radius
in 4U 1608-52 of 13 km (for masses 1.2-2.4 M$_\odot$). The best agreement with
the theoretical NS mass-radius relations is achieved for source distances in
the range 3.1-3.7 kpc. We expect that the radius limit will be 10 per cent
lower if spectral models including rapid rotation are used instead.
