  Aims. We present a cosmic shear analysis and data validation of 15 square
degree high-quality R-band data of the Garching-Bonn Deep Survey obtained with
the Wide Field Imager of the MPG/ESO 2.2m telescope. Methods. We measure the
two-point shear correlation functions to calculate the aperture mass
dispersion. Both statistics are used to perform the data quality control.
Combining the cosmic shear signal with a photometric redshift distribution of a
galaxy sub-sample obtained from two square degree of UBVRI-band observations of
the Deep Public Survey we determine constraints for the matter density Omega_m,
the mass power spectrum normalisation sigma_8 and the dark energy density
Omega_Lambda in the magnitude interval R in [21.5,24.5]. In this magnitude
interval the effective number density of source galaxies is n=12.5/sq. arcmin,
and their mean redshift is z_m=0.78. To estimate the posterior likelihood we
employ the Monte Carlo Markov Chain method. Results. Using the aperture mass
dispersion we obtain for the mass power spectrum normalisation sigma_8=0.80 +-
0.10 (1 sigma statistical error) at a fixed matter density Omega_m=0.30
assuming a flat universe with negligible baryon content and marginalising over
the Hubble parameter and the uncertainties in the fitted redshift distribution.
