  Ultra-massive hydrogen-rich white dwarfs (WDs) stars are expected to harbor
oxygen/neon cores resulting from semi-degenerate carbon burning when the
progenitor star evolves through the super asymptotic giant branch (SAGB) phase.
These stars are expected to be crystallized by the time they reach the ZZ Ceti
domain. We show that crystallization leads to a phase separation of oxygen and
neon in the core of ultra-massive WDs, which impacts markedly the pulsational
properties, thus offering a unique opportunity to study the processes of
crystallization and to infer the core chemical composition in WD stars.
