  We analyse the spectral energy distributions (SEDs) of the most distant
galaxies discovered with the Hubble Space telescope and from the COSMOS survey
and determine their physical properties, such as stellar age and mass, dust
attenuation, and star-formation rate. We use our SED fitting tool including the
effects of nebular emission to analyse three samples of z ~6-8 galaxies with
observed magnitudes J_AB~23 to 29. Our models cover a wide parameter space. We
find that the physical parameters of most galaxies cover a wide range of
acceptable values. Stellar ages, in particular, are not strongly constrained,
even for objects detected longward of the Balmer break. As already pointed out
earlier, the effects of nebular lines significantly affect the age
determinations of star-forming galaxies at z ~6-8. We find no need for stellar
populations with extreme metallicities or other non-standard assumptions (IMF,
escape fraction) to explain the observed properties of faint z-dropout
galaxies. Albeit with large uncertainties, our fit results show indications of
dust attenuation in some of the z ~6-8 galaxies, which have best-fit values of
A_V up to ~1. Furthermore, we find a possible trend of increasing dust
attenuation with galaxy mass, and a relatively large scatter in specific
star-formation rates, SFR/M*. The physical parameters of very high-z galaxies
may be more uncertain than indicated by previous studies. Dust attenuation
seems also to be present in some z ~6-8 galaxies, and may be correlated with
galaxy mass, as is also the case for SFR.
