B 1933+503, a dusty radio quasar at z > 2 - Implications for blank field sub-mm surveys?

We present a detailed mm-wave and optical study of the gravitational lens s ystem B 1933+503, discovered by Sykes et al. (1998) in the radio. This obje ct is probably the most complex lens system known, with 10 lensed component s within a radius of one arcsecond. It is potentially important as a probe of the Hubble constant, although no optical counterpart has thus far been o bserved down to I = 24.2. We have obtained new sub-millimetre detections at 450 mu m, 850 mu m and 1350 mu m. We have also constrained the possible du st emission from the proposed foreground lensing galaxy using a K-band adap tive optics image and CO(5-4) measurements. A lensing model is constructed, taking the foreground elliptical galaxy at z = 0.755 as the lensing mass. From this we derive a scenario from which to model the sub-millimetre emiss ion. Several arguments then point to the source in the B 1933+503 system ly ing above a redshift of 2. We speculate that unlensed relatives of this sou rce may constitute a sizable fraction of the 850 mu m source counts.