  We have investigated the origin of a sub-class of carbon-polluted white
dwarfs (DQ) originally identified as the ``hot DQ" white dwarfs. These objects
are relatively hot (10 000 < T_eff < 25 000 K), have markedly higher carbon
abundance (C-enriched), are more massive (M > 0.8 M_Sun) than ordinary DQs (M ~
0.6 M_Sun), and display high space velocities. Hence, despite their young
appearance their kinematic properties are those of an old white dwarf
population. The way out of this dilemma is to assume that they formed via the
merging of two white dwarfs. In this paper we examine the observed
characteristics of this population of ``C-enriched" DQ white dwarfs and confirm
that nearly half of the 63 known objects have kinematic properties consistent
with those of the Galactic thick disc or halo. We have also conducted
population synthesis studies and found that the merging hypothesis is indeed
compatible with observations. Studies of this sub-class of white dwarfs have
important implications for our understanding of Type Ia Supernovae (SNeIa),
commonly used to determine the expansion history of the universe, since the
same formation channel applies to both kinds of objects. Hence probing the
properties of these white dwarfs that failed to explode may yield important
constraints to the modelling of the mechanisms leading to a thermonuclear
runaway.
