  We analyse Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array
(PCA) data of a double-peaked burst from the low mass X-ray binary (LMXB) 4U
1636--536 that shows no evidence for photospheric radius expansion (PRE). We
find that the X-ray emitting area on the star increases with time as the burst
progresses, even though the photosphere does not expand. We argue that this is
a strong indication of thermonuclear flame spreading on the stellar surface
during such bursts. We propose a model for such double-peaked bursts, based on
thermonuclear flame spreading, that can qualitatively explain their essential
features, as well as the rarity of these bursts.
