  We have studied the penetration depth for a model of $YBa_{2}Cu_{3}O_{7}$
involving pairing both in the $CuO_{2}$ planes and in the CuO chains. In this
model pairing in the planes is due to an attractive interaction, while Coulomb
repulsion induces in the chains an order parameter with opposite sign. Due to
the anticrossing produced by hybridization between planes and chains, one
obtains a d-wave like order parameter which changes sign on a single sheet of
the Fermi surface and has nodes in the gap. We find that our model accounts
quite well for the anisotropy of the penetration depth and for the absolute
values. We reproduce fairly well the whole temperature dependence for both the
a and the b directions, including the linear dependence at low temperature. We
use a set of parameters which are all quite reasonable physically. Our results
for the c direction are also satisfactory, although the situation is less clear
both experimentally and theoretically.
