  The shape of the lightcurve during the rising phase of Type I X-ray bursts is
determined by many factors including the ignition latitude, the accretion rate,
and the rotation rate of the star. We develop a phenomenological model of the
burst rise process and show that simple measures of the burst morphology can be
robust diagnostics of ignition latitude and burning regime. We apply our
results to the large sample of bursts from the Low Mass X-ray Binary 4U
1636-536, and find evidence for off-equatorial ignition for many of the bursts.
We argue that such behaviour may be associated with the transition from
hydrogen to helium ignition at accretion rates a few percent of Eddington. We
show that this model can also explain variations in the detectability of burst
oscillations, and discuss the implications for other burst sources.
