  Coherent oscillations and the evolution of the X-ray spectrum during
thermonuclear X-ray bursts in accreting neutron-star X-ray binaries have been
studied intensively but separately. We analysed all the X-ray bursts of the
source 4U 1728-34 with the Rossi X-ray Timing Explorer. We found that the
presence of burst oscillations can be used to predict the behaviour of the
blackbody radius during the cooling phase of the bursts. If a burst shows
oscillations, during the cooling phase the blackbody radius remains more or
less constant for ~2 - ~8s, whereas in bursts that do not show oscillations the
blackbody radius either remains constant for more than ~2 - ~8s or it shows a
rapid (faster than ~2s) decrease and increase. Both the presence of burst
oscillations and the time-dependent spectral behaviour of the bursts are
affected by accretion rate. We also found that the rise time and convexity of
the bursts' light curve are different in bursts with and without oscillations
in 4U 1728--34. Bursts with oscillations have a short rise time (~0.5s) and
show both positive and negative convexity, whereas bursts without oscillations
have a long rise time (~1s) and mostly positive convexity. This is consistent
with the idea that burst oscillations are associated with off-equator ignition.
