Partial Breaking of N=1, D=10 Supersymmetry

We describe the spontaneous partial breaking of N=1,D=10 supersymmetry to N=(1,0),d=6 and its dimensionally-reduced versions in the framework of nonlinear realizations. The basic Goldstone superfield is N=(1,0),d=6 hypermultiplet superfield satisfying a nonlinear generalization of the standard hypermultiplet constraint. We interpret the generalized constraint as the manifestly worldvolume supersymmetric form of equations of motion of the type I super 5-brane in D=10. The related issues we address are a possible existence of brane extension of off-shell hypermultiplet actions, the possibility to utilize vector N=(1,0), d=6 supermultiplet as the Goldstone one, and the description of 1/4 breaking of N=1,D=11 supersymmetry.