  We examine Cr isotopes at the drip line, where surface effects related to the
existence of a weakly bound $s1/2$ state are known to be important and tightly
connected with the pairing phenomenon (anti-halo effect). For these
weakly-bound isotopes, we evaluate the ground state to ground state two-neutron
transfer probabilities within a mean-field-based approach. An important part of
the discussion is devoted to the analysis of several procedures that can be
employed to constrain the parameters of a phenomenological pairing interaction.
The parameters are first adjusted to reproduce the experimental gaps evaluated
with the five-point formula. This choice has however some consequences on the
evolution of the pairing correlations along the isotopic chains and, in
particular, at shell closures. Other procedures are then followed (adjustment
on a theoretical pairing gap at mid-shell and on the two-neutron separation
energies).
  For the transfer probabilities, we discuss the effects associated to
different choices of the spatial localization of the pairing interaction. We
indicate that the analysis of pair-transfer reactions for such cases (where the
last bound state is a low-$l$ state in a weakly bound nucleus) may improve our
understanding of two aspects: the spatial distribution of pairing correlations
in nuclei and the general problem of the persistence of pairing at the drip
lines.
