Weak lensing observations of the "dark" cluster MG2016+112
  We investigate the possible existence of a high-redshift (z=1) cluster of galaxies associated with the QSO lens system MG2016+112. From an ultra-deep R- and less deep V- and I-band Keck images and a K-band mosaic from UKIRT, we detect ten galaxies with colors consistent with the lensing galaxy within 225h^{-1} kpc of the z=1.01 lensing galaxy. This represents an overdensity of more than ten times the number density of galaxies with similar colors in the rest of the image. We also find a group of seven much fainter objects closely packed in a group only 27h^{-1} kpc north-west of the lensing galaxy. We perform a weak lensing analysis on faint galaxies in the R-band image and detect a mass peak of a size similar to the mass inferred from X-ray observations of the field, but located 64" northwest of the lensing galaxy.   From the weak lensing data we rule out a similar sized mass peak centered on the lensing galaxy at the 2 sigma level. 
D. Clowe (MPA), N. Trentham (IoA) and J. Tonry (IfA)