  The formation of collimated outflows or jets in planetary nebulae (PNe) is
not well understood. There is no evidence for active accretion disks in PNe
making it difficult to decide which of several proposed jet formation scenarios
may be correct. A handful of wide binary central stars of PNe are known to have
accreted carbon and slow neutron capture (s-process) enhanced material, the
immediate progenitors of barium stars, however no close binary analogues are
known to have passed through a common-envelope (CE) phase. Here we present
spectroscopy of The Necklace taken near lightcurve minimum that for the first
time reveals a carbon-rich (C/O > 1) companion, a carbon dwarf, in a post-CE
central star. As unevolved stars do not produce carbon, the chemical
enhancement of the secondary can only be explained by accretion from the
primary. Accretion most likely happened prior to the CE phase via wind
accretion as not enough material can be accreted during the short CE phase. The
pair of jets in The Necklace, which are observed to be older than the PN, are
therefore likely to have been launched from an accretion disk around the
companion during this early accretion phase. This discovery adds significant
weight to the emerging scenario that jets in post-CE PNe are primarily launched
by an accretion disk around a main-sequence companion before the CE phase.
