  Motivated by the recent observation by NICER of a type I X-ray burst from SAX
J1808.4-3658 with a distinct "pause" feature during its rise (Bult et al.
2019), we show that bursts which ignite in a helium layer underneath a
hydrogen-rich shell naturally give rise to such pauses, as long as enough
energy is produced to eject the outer layers of the envelope by super-Eddington
winds. The length of the pause is determined by the extent of the convection
generated after ignition, while the rate of change of luminosity following the
pause is set by the hydrogen gradient left behind by convection. Using the MESA
stellar evolution code, we simulate the accumulation, nuclear burning and
convective mixing prior to and throughout the ignition of the burst, followed
by the hydrodynamic wind. We show that the results are sensitive to the
treatment of convection adopted within the code. In particular, the efficiency
of mixing at the H/He interface plays a key role in determining the shape of
the lightcurve. The data from SAX J1808.4-3658 favors strong mixing scenarios.
Multidimensional simulations will be needed to properly model the interaction
between convection and nuclear burning during these bursts, which will then
enable a new way to use X-ray burst lightcurves to study neutron star surfaces.
