  Paring correlations in weakly bound nuclei on the edge of neutron drip line
is studied by using a three-body model. A density-dependent contact interaction
is employed to calculate the ground state of halo nuclei $^{6}$He and
$^{11}$Li, as well as a skin nucleus $^{24}$O. Dipole excitations in these
nuclei are also studied within the same model. We point out that the di-neutron
type correlation plays a dominant role in the halo nuclei $^{6}$He and
$^{11}$Li having the coupled spin of the two neutrons $S$=0, while the
correlation similar to the BCS type is important in $^{24}$O. Contributions of
the spin $S$=1 and S=0 configurations are separately discussed in the low
energy dipole excitations.
