  We report the discovery of two bright arcs in what turns out to be the
brightest X--ray cluster in the ROSAT band ever observed, RXJ1347.5-1145. Its
luminosity is $(6.2\pm0.6) \cdot10^{45}$erg s$^{-1}$ (in the range
0.1--2.4~keV). The arcs are most probably gravitationally lensed images of
background galaxies. They were found serendipitously during our ongoing
large--scale redshift survey of X--ray clusters detected by the ROSAT All Sky
Survey. The arcs are almost opposite to each other with respect to the cluster
centre, with a distance from it of about $35''$ ($=240 h^{-1}_{50}$ kpc), a
radius that enables the probing of a rather large cluster volume. In this
Letter we limit ourselves to the discussion of the general optical and X--ray
features of this cluster and to the potential implications of the gravitational
arcs. A more detailed discussion of the different mass estimates and of the
cosmological implications for this exceptional object are left for future work
based on more accurate optical and X--ray data, which are currently being
collected.
