  The sub-leading power of the scattering amplitude for deeply-virtual Compton
scattering (DVCS) off the nucleon contains leading-twist and twist-3
generalized parton distributions (GPDs). We point out that in DVCS, at twist-3
accuracy, one cannot address any individual twist-3 GPD. This complication
appears on top of the deconvolution issues familiar from the twist-2 DVCS
amplitude. Accessible are exclusively linear combinations involving both vector
and axial-vector twist-3 GPDs. This implies, in particular, that the (kinetic)
orbital angular momentum of quarks can hardly be constrained by twist-3 DVCS
observables. Moreover, using the quark-target model, we find that twist-3 GPDs
can be discontinuous. The discontinuities however cancel in the DVCS amplitude,
which further supports the hypothesis of factorization at twist-3 accuracy.
