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 " sout heast of RGB 1745+398 (z = 0.267) during deep imaging of X-ray and radio se lected BL Lacertae candidates. The spectrum of the are resembles that of a late-type galaxy with three spectral features ([O II] emission, Mg Ir absor ption and 4000 Angstrom break) that indicate z = 1.057. We interpret the ar e as a gravitationally enhanced image of a background galaxy and the observ ed velocity splitting of the [O II] line (350 km s(-1)) as due to its rotat ion, although we cannot rule out the possibility that the source consists o f a pair of background galaxies. We find the host galaxy of RGB 1745+398 to be the brightest duster member (M-R = -24.9) of a moderately massive clust er that most likely provides the lensing mass. From simple considerations w e obtain M similar to 1.3 x 10(13) M-. and M/L-B similar to 60 for the cent ral part (r < 40 h(50)(-1) kpc) of the cluster.