Abstract
For a triplet superconductor in a magnetic field, a phase is predicted in which spins in only one direction are paired, as in the Al phase of 3He. The upper critical field Hc2(T) separating this phase from the normal state is larger than the Hc2 one would have in the absence of the Pauli paramagnetism responsible for the new phase. The effect depends on the derivative of the density of states at the Fermi energy and is reduced but not destroyed by spin-orbit coupling. Comparison is made to the superconducting phase diagram of UPt3 measured by V. Muller et. al. [Phys. Rev. Lett. 58, 1224 (1987).]