  Hierarchical triple systems are common among field stars yet their long-term
evolution is poorly understood theoretically. In such systems Kozai cycles can
be induced in the inner binary system during which the inner orbit eccentricity
and the inclination between both binary orbits vary periodically. These cycles,
combined with tidal friction and gravitational wave emission, can significantly
affect the inner binary evolution. To investigate these effects quantitatively
we perform a population synthesis study of triple systems and focus on
evolutionary paths that lead to mergers of carbon-oxygen (CO) white dwarfs
(WDs), which constitute an important candidate progenitor channel for type Ia
supernovae (SNe Ia). We approach this problem by Monte Carlo sampling from
observation-based distributions of systems within the primary mass range 1.0 -
6.5 M_Sun and inner orbit semi-major axes a_1 and eccentricities e_1 satisfying
a_1 (1-e_1^2) > 12 AU, i.e. non-interacting in the absence of a tertiary
component. We evolve these systems by means of a newly developed algorithm that
couples secular triple dynamics with an existing binary population synthesis
code. We find that the tertiary significantly alters the inner binary evolution
in about 24% of all sampled systems. In particular, we find several channels
leading to CO WD mergers. Amongst these is a novel channel in which a collision
occurs in wide inner binary systems as a result of extremely high
eccentricities induced by Kozai cycles. With assumptions on which CO WD mergers
lead to a SN Ia explosion we estimate the SNe Ia delay time distribution
resulting from triples and compare to a binary population synthesis study and
to observations. Although we find that our triple rate is low, we have
determined a lower limit of the triple-induced SNe Ia rate and further study is
needed that includes triples with initially tighter inner orbits.
