  We analyse near-infrared HST/NICMOS F110W (J) and F160W (H) band photometry
of a sample of 27 i'-drop candidate z~6 galaxies in the central region of the
HST/ACS Ultra Deep Field (HUDF). The infrared colours of the 20 objects not
affected by near neighbours are consistent with a high redshift interpretation.
This suggests that the low redshift contamination of this i'-drop sample is
smaller than that observed at brighter magnitudes where values of 10-40% have
been reported. The J-H colours are consistent with a slope flat in f_nu (f_lam
\propto lam^{-2}), as would be expected for an unreddened starburst. There is,
however, evidence for a marginally bluer spectral slope (f_lam propto
lam^{-2.2) which is perhaps indicative of an extremely young starburst (~10 Myr
old) or a top heavy initial mass function and little dust. The low levels of
contamination, median photometric redshift of z~6.0 and blue spectral slope,
inferred using the near-infrared data, supports the validity of the assumptions
in our earlier work in estimating the star formation rates and, that the
majority of the i-drop candidates galaxies lie at z~6.
