  Bound states made from two triplet excitations on the Shastry-Sutherland
(ShaSu) lattice are investigated. Based on the perturbative unitary
transformation by flow equations quantitative properties like dispersions and
qualitative properties like symmetries are determined. The high order results
(up to (J_2/J_1)^{14}) permit to fix the parameters of SrCu_2(BO_3)_2
precisely: J_1=6.16(10)meV, x:=J_2/J_1=0.603(3), J_\perp=1.3(2)meV. At the
border of the magnetic Brillouin zone (MBZ) a general double degeneracy is
derived. An unexpected instability in the triplet channel at x=0.63 indicates a
first order transition towards a triplet condensate, related to classical
helical order.
