  Using HST WFPC2 V- and R-band data of the z=0.37 cluster MS1512+36 we show
that the z=2.72 `protogalaxy' cB58 is not extraordinarily luminous
intrinsically but lensed into a gravitational fold arc by the cluster. The arc
has a surface brightness weighted axis ratio of 1:7, is marginally resolved in
width and about 3 arcsec long. Its counterimage was identified and found to be
very compact r(1/2) = 2.4-4.0 / h50 kpc in a q0=0.05 cosmology). In addition,
we found three further multiple image systems, one with five and two with three
images each. The positions of the multiple images is explained by modelling the
light deflection caused by the cluster and the cD-galaxy with elliptical
isothermal potentials. Since the multiple images are within 8 arcsec around the
cD galaxy, a cluster core radius - cluster velocity dispersion degeneracy
arises. The cluster velocity dispersion and the core radius are limited to
540-670 km/s and 5 arcsec - 11\arcsec, respectively, and the brightness of the
unlensed counterimage of cB58 is about 23.9 +- 0.3 (R-band), corresponding to a
magnification and extinction corrected restframe-B band absolute magnitude of
-24.75 +- 0.7 mag. The redshifts of the sources of the remaining three multiple
image systems are predicted to be similar to that of cB58. That part of the
source of cB58 which is mapped into the arc is reconstructed and its
magnification is found to be mu(arc) > 50. This large magnification explains at
least some of the untypical spectroscopic properties of cB58, e.g. that the
star formation rate seems to be high and uniform and to take place in a large
area.
