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 ( \delta \geq 0 ^ { \circ } ) and at high Galactic latitudes ( |b| \geq 20 ^ { \circ } ) .
It comprises 100 X-ray selected clusters of galaxies with measured redshifts z \leq 0.3 ( as well as seven more at z > 0.3 ) and total fluxes between 2.8 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } and 4.4 \times 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 .

An electronic version of the eBCS can be obtained from the following URL : www.ifa.hawaii.edu/ \sim ebeling/clusters/BCS.html .