1999A&A...343...81N

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 ([OII] emission, MgII absorption and 4000Å 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 [OII] line (350km/s) 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∼1.3x10¹³M_☉ and M/L_B∼60 for the central part (r<40h₅₀^–1kpc) of the cluster.