B1933+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 system B1933+503, discovered by Sykes et al.(1998) in the radio. This object is probably the most complex lens system known, with 10 lensed components 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 observed down to I=24.2. We have obtained new sub-millimetre detections at 450, 850 and 1350 microns. We have also constrained the possible dust emission from the proposed foreground lensing galaxy using a K-band adaptive 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 emission. Several arguments then point to the source in the B1933+503 system lying above a redshift of 2. We speculate that unlensed relatives of this source may constitute a sizable fraction of the 850 micron source counts.