  Mergers of compact object binaries are one of the most powerful sources of
gravitational waves (GWs) in the frequency range of second-generation
ground-based gravitational wave detectors (Advanced LIGO and Virgo). Dynamical
simulations of young dense star clusters (SCs) indicate that ~27 per cent of
all double compact object binaries are members of hierarchical triple systems
(HTs). In this paper, we consider 570 HTs composed of three compact objects
(black holes or neutron stars) that formed dynamically in N-body simulations of
young dense SCs. We simulate them for a Hubble time with a new code based on
the Mikkola's algorithmic regularization scheme, including the 2.5
post-Newtonian term. We find that ~88 per cent of the simulated systems develop
Kozai-Lidov (KL) oscillations. KL resonance triggers the merger of the inner
binary in three systems (corresponding to 0.5 per cent of the simulated HTs),
by increasing the eccentricity of the inner binary. Accounting for KL
oscillations leads to an increase of the total expected merger rate by ~50 per
cent. All binaries that merge because of KL oscillations were formed by
dynamical exchanges (i.e. none is a primordial binary) and have chirp mass >20
Msun. This result might be crucial to interpret the formation channel of the
first recently detected GW events.
