  Time-resolved spectra during the cooling phase of thermonuclear X-ray bursts
in low-mass X-ray binaries (LMXBs) can be used to measure the radii and masses
of neutron stars. We analyzed ~ 300 bursts of the LMXB 4U 1636-53 using data
from the Rossi X-ray Timing Explorer. We divided the bursts in three groups,
photospheric radius expansion (PRE), hard non-PRE and soft non-PRE bursts,
based on the properties of the bursts and the state of the source at the time
of the burst. For the three types of bursts, we found that the average relation
between the bolometric flux and the temperature during the cooling phase of the
bursts is significantly different from the canonical $F \propto T^4$ relation
that is expected if the apparent emitting area on the surface of the neutron
star remains constant as the flux decreases during the decay of the bursts. We
also found that a single power law cannot fit the average flux-temperature
relation for any of the three types of bursts, and that the flux-temperature
relation for the three types of bursts is significantly different. Finally, for
the three types of bursts, the temperature distribution at different flux
levels during the decay of the bursts is significantly different. From the
above we conclude that hard non-PRE bursts ignite in a hydrogen-rich
atmosphere, whereas for soft non-PRE and PRE bursts the fuel is helium-rich. We
further conclude that the metal abundance in the neutron star atmosphere
decreases as the bursts decay, probably because the heavy elements sink faster
in the atmosphere than H and He.
