Discovery of a blue arc near the BL Lacertae object RGB 1745+398

We have made a serendipitous discovery of a blue arclike structure 8'' southeast of RGB 1745+398 (z = 0.267) during deep imaging of X-ray and radio selected BL Lacertae candidates. The spectrum of the arc resembles that of a late-type galaxy with three spectral features ([O Ii] emission, Mg Ii absorption and 4000 Angstroms break) that indicate z = 1.057. We interpret the arc as a gravitationally enhanced image of a background galaxy and the observed velocity splitting of the [O Ii] line (350 km s(-1) ) as due to its rotation, although we cannot rule out the possibility that the source consists of a pair of background galaxies. We find the host galaxy of RGB 1745+398 to be the brightest cluster member (M_R = -24.9) of a moderately massive cluster that most likely provides the lensing mass. From simple considerations we obtain M ~ 1.3 x 10(13) M_⊙ and M/L_B ~ 60 for the central part (r < 40\ h₅₀(-1) kpc) of the cluster. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations collected at the German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy