  The zero temperature in plane penetration depth in an untwinned single
crystal of optimally doped YBa_2Cu_3O_6.93 is highly anisotropic. This fact has
been interpreted as evidence that a large amount of the condensate resides in
the chains. On the other hand, the temperature dependence of
lambda_a(T)/lambda_a(0) and lambda_b(T)/lambda_b(0) (where the b-direction is
along the chains) are not very different. This constrains theories and is, in
particular, difficult to understand within a proximity model with d-wave
pairing only in the CuO_2 plane and none on the CuO chains but instead supports
a more three dimensional models with interplane interactions.
