= Little group with mixed string boundary conditions
The symmetry used to classify an <open string> must preserve its endpoint boundary background as well as its <momentum>. A <Neumann-Dirichlet open-string boundary condition> breaks target Lorentz transformations mixing ND and common directions with <Neumann boundary conditions>. For a D1–D25 pair with 24 ND directions, the common worldvolume has Lorentz group $SO(1,1)$, whose connected massive <little group> is trivial. The transverse $SO(24)$ remains as an internal rotation group. Thus a massive 24-component oscillator multiplet is not an $SO(25)$ vector of the full 26-dimensional massive little group; a vector of that full group would have 25 components.
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