  We report optical spectroscopic observations of RX J0018.8+1602, a ROSAT
X-ray source proposed to be an intermediate redshift cluster of galaxies. Our
observations confirm the identification of RX J0018.8+1602 and provide
measurements of its mean radial velocity (z=0.5406 +/- 0.0006) and velocity
dispersion (sigma_p = 200 [+110, -80] km/s). This is the second poor cluster
that has been found to be a companion to CL 0016+16 (z=0.5455), the other one
being RX J0018.3+1618 (z=0.5506). The 0.2-2 keV band source-frame X-ray
luminosity summed over both companion clusters is 5E44 ergs/s, which is a
significant fraction, ~23%, of the X-ray luminosity of the main cluster. The
companions are located at angular distances of 10 arcmin to 25 arcmin (minimum
physical scales of 5 Mpc to 12 Mpc) from CL 0016+16 and we propose that they
represent a new large-scale component of the X-ray emission from clusters of
galaxies. Similar low X-ray luminosity poor clusters surrounding nearby Abell
clusters can explain the excess power observed in the angular cross-correlation
function between Abell clusters and the X-ray background on inferred physical
scales of 14--20 Mpc (Soltan et al. 1996).
