  Within the project Galactic Cold Cores we are carrying out Herschel
photometric observations of cold interstellar clouds detected with the Planck
satellite. The three fields observed as part of the Herschel science
demonstration phase (SDP) provided the first glimpse into the nature of these
sources. We examine the properties of the dust emission within the fields. We
determine the dust sub-millimetre opacity, look for signs of spatial variations
in the dust spectral index, and estimate how the apparent variations of the
parameters could be affected by different sources of uncertainty. We use the
Herschel observations where the zero point of the surface brightness scale is
set with the help of the Planck satellite data. We derive the colour
temperature and column density maps of the regions and determine the dust
opacity by a comparison with extinction measurements. By simultaneously fitting
the colour temperature and the dust spectral index values we look for spatial
variations in the apparent dust properties. With a simple radiative transfer
model we estimate to what extent these can be explained by line-of-sight
temperature variations, without changes in the dust grain properties. The
analysis of the dust emission reveals cold and dense clouds that coincide with
the Planck sources and confirm those detections. The derived dust opacity
varies in the range kappa(250um) ~ 0.05-0.2 cm^2/g, higher values being
observed preferentially in regions of high column density. The average dust
spectral index beta is ~ 1.9-2.2. There are indications that beta increases
towards the coldest regions. The spectral index decreases strongly near
internal heating sources but, according to radiative transfer models, this can
be explained by the line-of-sight temperature variations without a change in
the dust properties.
