Context : 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 .
The aim of the project is to derive the physical properties of the full cold core population revealed by Planck .

Aims : We examine the properties of the dust emission within the three fields observed during the SDP .
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 .

Methods : 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 .

Results : 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 ( 250 \mu { m } ) \sim 0.05–0.2 cm ^ { 2 } g ^ { -1 } , higher values being observed preferentially in regions of high column density .
The average dust spectral index \beta is \sim 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 .

Conclusions :