While the rp-process in X-ray bursts may have difficulty bypassing the Ge waiting point, certainly in X-ray pulsars where the rp -process is stable, instability toward alpha decay places an upper limit near A = 100 on the mass that can be reached through continuous burning. The exact limit is a matter presently under investigation; Te are known to undergo alpha decay whereas Sb is proton-unbound. Even before the limit near A = 100 is reached, the proton flux is thought to considerably decrease and thus slow down the rp -process, before low capture rate and a cycle of transmutations between isotopes of tin, antimony, and tellurium upon further proton capture terminate it altogether. However, it has been shown that if there are episodes of cooling or mixing of previous ashes into the burning zone, material as heavy as Xe can be created.