  Gaia's exquisite parallax measurements allowed for the discovery and
characterization of the Q branch in the Hertzsprung-Russell diagram, where
massive C/O white dwarfs (WDs) pause their dimming due to energy released
during crystallization. Interestingly, the fraction of old stars on the Q
branch is significantly higher than in the population of WDs that will become Q
branch stars or that were Q branch stars in the past. From this, Cheng et al.
inferred that ~6% of WDs passing through the Q branch experience a much longer
cooling delay than that of standard crystallizing WDs. Previous attempts to
explain this cooling anomaly have invoked mechanisms involving super-solar
initial metallicities. In this paper, we describe a novel scenario in which a
standard composition WD merges with a subgiant star. The evolution of the
resulting merger remnant leads to the creation of a large amount of 26Mg,
which, along with the existing 22Ne, undergoes a distillation process that can
release enough energy to explain the Q branch cooling problem without the need
for atypical initial abundances. The anomalously high number of old stars on
the Q branch may thus be evidence that mass transfer from subgiants to WDs
leads to unstable mergers.
