16 Feb. 2007: figures 5 and 6 have been replaced. In the original figures the y-axis scale for left and right plots was different and now the same scale is used for better visibility. The values of the data points and theory curves have not changed.
16. Feb. 2007: Comparisons using updated MRST NNLO calculation F2ccF2bb
Comparisons with a massless calculation (H1PDF2000): F2cc; F2bb.