Bound States of Black Holes and Other $P$-Branes
  In the process of identifying heterotic and Type $II$ BPS string states with extremal dilaton black holes, it has been suggested that solutions with scalar/Maxwell parameters $a=\sqrt{3}$, $1$, $1/\sqrt{3}$ and $0$ correspond to $1-$, $2-$, $3-$ and $4$-particle bound states at threshold. (For example, the Reissner-Nordstrom black hole is just a superposition of four Kaluza-Klein black holes). Here we show that not only the masses, electric charges and magnetic charges but also the spins and supermultiplet structures of the string states are consistent with this interpretation. Their superspin $L$ corresponds to the Kerr-type angular momentum and hence only the $L=0$ elementary BPS states are black holes. Moreover, these results generalize to super $p$-branes in $D$-dimensions. By constructing multi-centered $p$-brane solitons, the new super $p$-branes found recently with various values of $a^2=\Delta-2(p+1)(D-p-3)/(D-2)$ are seen to be bound states of the fundamental ones with $\Delta=4$. 
M.J. Duff and J. Rahmfeld