  The influence of pairing correlations on the neutron root mean square (rms)
radius of nuclei is investigated in the framework of self-consistent Skyrme
Hartree-Fock-Bogoliubov calculations. The continuum is treated appropriately by
the Green's function techniques. As an example the nucleus $^{124}$Zr is
treated for a varying strength of pairing correlations. We find that, as the
pairing strength increases, the neutron rms radius first shrinks, reaches a
minimum and beyond this point it expands again. The shrinkage is due to the the
so-called `pairing anti-halo effect', i. e. due to the decreasing of the
asymptotic density distribution with increasing pairing. However, in some
cases, increasing pairing correlations can also lead to an expansion of the
nucleus due to a growing occupation of so-called `halo' orbits, i.e. weakly
bound states and resonances in the continuum with low-$\ell $ values. In this
case, the neutron radii are extended just by the influence of pairing
correlations, since these `halo' orbits cannot be occupied without pairing. The
term `anti-halo effect' is not justified in such cases. For a full
understanding of this complicated interplay self-consistent calculations are
necessary.
