  We present a low-flux extension of the X-ray selected ROSAT Brightest Cluster
Sample (BCS) published in Paper I of this series. Like the original BCS and
employing an identical selection procedure, the BCS extension is compiled from
ROSAT All-Sky Survey (RASS) data in the northern hemisphere (dec > 0 deg) and
at high Galactic latitudes (|b| > 20 deg). It comprises 100 X-ray selected
clusters of galaxies with measured redshifts z < 0.3 (as well as seven more at
z > 0.3) and total fluxes between 2.8 x 10-12 erg cm-2 s-1 and 4.4 x 10-12 erg
cm-2 s-1 in the 0.1-2.4 keV band (the latter value being the flux limit of the
original BCS). The extension can be combined with the main sample published in
1998 to form the homogeneously selected extended BCS (eBCS), the largest and
statistically best understood cluster sample to emerge from the ROSAT All-Sky
Survey to date.
  The nominal completeness of the combined sample (defined with respect to a
power law fit to the bright end of the BCS log N-log S distribution) is
relatively low at 75 per cent (compared to 90 per cent for the high-flux sample
of Paper I). However, just as for the original BCS, this incompleteness can be
accurately quantified, and thus statistically corrected for, as a function of
X-ray luminosity and redshift.
  In addition to its importance for improved statistical studies of the
properties of clusters in the local Universe, the low-flux extension of the BCS
is also intended to serve as a finding list for X-ray bright clusters in the
northern hemisphere which we hope will prove useful in the preparation of
cluster observations with the next generation of X-ray telescopes such as
Chandra or XMM-Newton.

Astrophysics