  Following the recent work of Caves, Fuchs, and Rungta [Found. of Phys. Lett.
{\bf 14} (2001) 199], we discuss some entanglement properties of two-rebits
systems. We pay particular attention to the relationship between entanglement
and purity. In particular, we determine (i) the probability densities for
finding pure and mixed states with a given amount of entanglement, and (ii) the
mean entanglement of two-rebits states as a function of the participation
ratio.
