  This Letter presents the serendipitous discovery of a large arc in an X-ray
selected cluster detected in the Rosat North Ecliptic Pole (NEP) survey. The
cluster, associated with Abell 2280, is identified as the optical counterpart
of the X-ray source RXJ 1743.5+6341. This object is a medium--distant z=0.326
and luminous (L_(0.5-2 kev) = 5.06x10**44 erg/s) cluster dominated by a central
bright galaxy. The arc is located ~ 14 arcsec to the North-West of the cD and
it is detected in the R and I bands but not in the B band. Photometric and
spectroscopic observations of the cluster and of the arc are presented. Other
similar discoveries in the course of imaging and spectroscopic surveys of Rosat
are expected in the future.
