  Studies of the spin and parity quantum numbers of the Higgs boson in the
$WW^\ast \to e \nu \mu \nu$ final state are presented, based on proton-proton
collision data collected by the ATLAS detector at the Large Hadron Collider,
corresponding to an integrated luminosity of 20.3 fb$^{-1}$ at a centre-of-mass
energy of $\sqrt{s}=8$ TeV. The Standard Model spin-parity $J^{CP} = 0^{++}$
hypothesis is compared with alternative hypotheses for both spin and CP. The
case where the observed resonance is a mixture of the Standard-Model-like Higgs
boson and CP-even ($J^{CP} = 0^{++}$) or CP-odd ($J^{CP} = 0^{+-}$) Higgs boson
in scenarios beyond the Standard Model is also studied. The data are found to
be consistent with the Standard Model prediction and limits are placed on
alternative spin and CP hypotheses, including CP mixing in different scenarios.
