We present ground-based and HST optical observations of the optical
transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and
090618, both at a redshift of z = 0.54. For GRB 060729, bumps are seen
in the optical light curves (LCs), and the late-time broadband spectral
energy distributions (SEDs) of the OT resemble those of local type Ic
supernovae (SNe). For GRB 090618, the dense sampling of our optical
observations has allowed us to detect well-defined bumps in the optical
LCs, as well as a change in colour, that are indicative of light coming
from a core-collapse SN. The accompanying SNe for both events are
individually compared with SN1998bw, a known GRB-supernova, and SN1994I,
a typical type Ic supernova without a known GRB counterpart, and in both
cases the brightness and temporal evolution more closely resemble
SN1998bw. We also exploit our extensive optical and radio data for GRB
090618, as well as the publicly-available SWIFT -XRT data, and discuss
the properties of the afterglow at early times. In the context of a
simple jet-like model, the afterglow of GRB 090618 is best explained by
the presence of a jet-break at t-to > 0.5 days. We then compare the
rest-frame, peak V -band absolute magnitudes of all of the GRB and X-Ray
Flash (XRF)-associated SNe with a large sample of local type Ibc SNe,
concluding that, when host extinction is considered, the peak magnitudes
of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of
non-GRB/XRF SNe.
