  Accreting neutron stars exhibit Type I X-ray bursts from both frequent
hydrogen/helium flashes as well as rare carbon flashes. The latter
(superbursts) ignite in the ashes of the former. Hydrogen/helium bursts,
however, are thought to produce insufficient carbon to power superbursts.
Stable burning could create the required carbon, but this was predicted to only
occur at much larger accretion rates than where superbursts are observed. We
present models of a new steady-state regime of stable hydrogen and helium
burning that produces pure carbon ashes. Hot CNO burning of hydrogen heats the
neutron star envelope and causes helium to burn before the conditions of a
helium flash are reached. This takes place when the mass accretion rate is
around 10% of the Eddington limit: close to the rate where most superbursts
occur. We find that increased heating at the base of the envelope sustains
steady-state burning by steepening the temperature profile, which increases the
amount of helium that burns before a runaway can ensue.
