  The search for the progenitors of six core-collapse supernovae (CCSNe) in
archival HST WFPC2 pre-explosion imaging is presented here. These SNe are
1999an, 1999br, 1999ev, 2000ds, 2000ew and 2001B. Post-explosion imaging of the
SNe, with the HST ACS/WFC, has been utilised with the technique of differential
astrometry to identify the progenitor locations on the pre-explosion imaging.
SNe 1999br, 1999ev, 2000ew and 2001B are recovered in late time imaging and an
estimate of the progenitor location on the pre-explosion imaging, with
sub-pixel accuracy, has been made. Only the progenitor of the type II-P SN
1999ev has been recovered, on pre-explosion F555W imaging, at a $4.8\sigma$
significance level. Assuming a red supergiant progenitor, the pre-explosion
observation is consistent with $M_{ZAMS}=15-18M_{\odot}$. The progenitors of
the other five SNe were below the $3\sigma$ detection threshold of the
pre-explosion observations. The detection thresholds were translated to mass
limits for the progenitors by comparison with stellar evolution models.
Pre-explosion observations of the peculiarly faint SN 1999br limit the mass of
a red supergiant progenitor to $M_{ZAMS} < 12M_{\odot}$. Analysis has been
extended, from previous studies, to include possible detections of
high-$\mathrm{T_{eff}}$, high-mass stars by conducting synthetic photometry of
model Wolf-Rayet star spectra. The mass limits for the type II-P SNe 1999an and
1999br are consistent with the previously determined mass limits for this type
of SN. The detection limits for the progenitors of the type Ibc SNe (2000ds,
2000ew and 2001B) do not permit differentiation between high-mass WR
progenitors or low mass progenitors in binaries.
