Geometry of D1-D5-P bound states
  Supersymmetric solutions of 6-d supergravity (with two translation symmetries) can be written as a hyperkahler base times a 2-D fiber. The subset of these solutions which correspond to true bound states of D1-D5-P charges give microstates of the 3-charge extremal black hole. To understand the characteristics shared by the bound states we decompose known bound state geometries into base-fiber form. The axial symmetry of the solutions make the base Gibbons-Hawking. We find the base to be actually `pseudo-hyperkahler': The signature changes from (4,0) to (0,4) across a hypersurface. 2-charge D1-D5 geometries are characterized by a `central curve' $S^1$; the analogue for 3-charge appears to be a hypersurface that for our metrics is an orbifold of $S^1\times S^3$. 
Stefano Giusto, Samir D. Mathur