  We systematically reanalyzed all Gamma-Ray Burst (GRB) afterglow data
published through the end of 2002, in an attempt to detect the predicted
supernova light component and to gain statistical insight on its
phenomenological properties. We fit the observed photometric light curves as
the sum of an afterglow, an underlying host galaxy, and a supernova component.
The latter is modeled using published multi-color light curves of SN 1998bw as
a template. The total sample of afterglows with established redshifts contains
21 bursts (GRB 970228 - GRB 021211). For nine of these GRBs a weak supernova
excess (scaled to SN 1998bw) was found, what makes this to one of the first
samples of high-z core collapse supernovae. Among this sample are all bursts
with redshifts less than ~0.7. These results strongly support the notion that
in fact all afterglows of long-duration GRBs contain light from an associated
supernova. A statistics of the physical parameters of these GRB-supernovae
shows that SN 1998bw was at the bright end of its class, while it was not
special with respect to its light curve shape. Finally, we have searched for a
potential correlation of the supernova luminosities with the properties of the
corresponding bursts and optical afterglows, but we have not found such a
relation.
