  We calculate the rapid proton (rp) capture process of hydrogen burning on the
surface of an accreting neutron star with an updated reaction network that
extends up to Xe, far beyond previous work. In both steady-state nuclear
burning appropriate for rapidly accreting neutron stars (such as the magnetic
polar caps of accreting X-ray pulsars) and unstable burning of Type I X-ray
bursts, we find that the rp process ends in a closed SnSbTe cycle. This
prevents the synthesis of elements heavier than Te and has important
consequences for X-ray burst profiles, the composition of accreting neutron
stars, and potentially galactic nucleosynthesis of light p nuclei.
