  Many white dwarf stars show signs of having accreted smaller bodies, implying
that they may host planetary systems. A small number of these systems contain
gaseous debris discs, visible through emission lines. We report a stable
123.4min periodic variation in the strength and shape of the CaII emission line
profiles originating from the debris disc around the white dwarf
SDSSJ122859.93+104032.9. We interpret this short-period signal as the signature
of a solid body held together by its internal strength.
