  The superconductor Sr_2RuO_4 is widely believed to be a spin triplet system
with a chiral order parameter analogous to the A phase of superfluid helium-3.
The best evidence for this pairing state is that the Knight shift or spin
susceptibility measured in neutron scattering is constant below T_c, unlike in
a spin-singlet superconductor. The original Knight shift and neutron scattering
measurements were performed for magnetic fields aligned in the ruthenate a-b
plane. These would be consistent with a triplet d-vector d{k} aligned along the
c-axis. However recently the Knight shift for fields along c was also found to
be constant below T_c, which is not expected for this symmetry state. In this
paper we show that while spin-orbit interaction stabilises the c-axis oriented
d-vector, it is possible that only a very small external B field may be
sufficient to rotate the d-vector into the a-b plane. In this case the triplet
pairing model remains valid. We discuss characteristics of the transition and
the prospects to detect it in thermodynamic quantities.
