  By comparing two age indicators of high-mass white dwarfs derived from Gaia
data, two discoveries have been made recently: one is the existence of a
cooling anomaly that produces the Q branch structure on the
Hertzsprung--Russell diagram, the other is the existence of double-white-dwarf
merger products. The former poses a challenge for white dwarf cooling models,
and the latter has implications on binary evolution and type-Ia supernovae.
