of $SO(6,6)$ turn out to be carried by $D$-branes, and as a result they give masses proportional to $1/\lambda$.

The BPS Condition
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The $N$-extended 4d supersymmetry algebra (in 2-component notation) includes the anticommutator $$\{ Q^I_{\alpha}, Q^J_{\beta} \} = \epsilon_{\alpha\beta} Z^{I J}.$$ The $N(N-1)/2$ central charges $Z^{IJ} = - Z^{JI}$ are complex numbers whose real and imaginary parts give the electric and magnetic charges associated with the $N(N-1)/2$ $U(1)$ gauge fields in the $N$-extended 4d supergravity multiplet. The supersymmetry algebra implies that the mass of any state is bounded below by its central charges. This bound, known as the Bogomol’nyi bound, is very important. When the mass of a state attains the minimum value allowed for given charges (and moduli), the state is said to be BPS saturated. BPS states belong to smaller representations of the algebra than are possible when the bound is not saturated. There are actually several possibilities for how this can be achieved. To explain this, it is convenient to make an $SO(N)$ change of basis such that (in the case of $N=4$, for example) $$Z = \left(\begin{array}{cccc}0 & Z_1 & 0 & 0\cr -Z_1& 0 & 0 & 0 \cr 0& 0 & 0 & 