  Electromagnetic properties of diquarks are investigated in the framework of a
color-octet Nambu-Jona-Lasinio model, which describes baryons as bound states
of diquarks and quarks. We calculate the electromagnetic form factors of scalar
and axial vector diquark bound states using the gauge--invariant proper--time
regularization. The nucleon charge radii and magnetic moments are estimated in
a simple additive diquark--quark model. This picture reproduces the qualitative
features such as the negative charge radius of the neutron. Within this model
axial vector diquarks are seen to be important.
