  We examine the so(2,D-1) WZW model at the subcritical level -(D-3)/2. It has
a singular vacuum vector at Virasoro level 2. Its decoupling constitutes an
affine extension of the equation of motion of the (D+1)-dimensional conformal
particle, i.e. the scalar singleton. The admissible (spectrally flowed)
representations contain the singleton and its direct products, consisting of
massless and massive particles in AdS_D. In D=4 there exists an extended model
containing both scalar and spinor singletons of sp(4). Its realization in terms
of 4 symplectic-real bosons contains the spinor-oscillator constructions of the
4D singletons and their composites. We also comment on the prospects of
relating gauged versions of the models to the phase-space quantization of
partonic branes and higher-spin gauge theory.
