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The T2K experiment has provided the first hint for the best-fit value for theleptonic CP phase $\delta_{CP} \sim -90^\circ$ from neutrino data. This is nowcorroborated by the NO$\nu$A neutrino runs. We study the implications forneutrino mass hierarchy and octant of $\theta_{23}$ in the context of this dataassuming that the true value of $\delta_{CP}$ in nature is $-90^\circ$. Basedon simple arguments on degeneracies in the probabilities we show that a clearsignal of $\delta_{CP}=-90^\circ$ coming from T2K neutrino (antineutrino) datais only possible if the true hierarchy is normal and the true octant is higher(lower). Thus if the T2K neutrino and antineutrino data are fitted separatelyand both give the true value of $\delta_{CP}=-90^\circ$, this will imply thatnature has chosen the true hierarchy to be normal and $\theta_{23} \approx45^\circ$. However we find that the combined fit of neutrino and antineutrinodata will still point to true hierarchy as normal but the octant of$\theta_{23}$ will remain undetermined. We do our analysis for both, thecurrent projected exposure ($7.8 \times 10^{21}$ pot) and planned extendedexposure ($20 \times 10^{21}$ pot). We also present the CP discovery potentialof T2K emphasizing on the role of antineutrinos. We find that one of the maincontribution of the antineutrino data is to remove the degenerate solutionswith the wrong octant. Thus the antineutrino run plays a more significant rolefor those hierarchy-octant combinations for which this degeneracy is present.If this degeneracy is absent, then only neutrino run gives a better result forfixed $\theta_{13}$. However if we marginalize over $\theta_{13}$ then,sensitivity corresponding to mixed run can be better than pure neutrino run.

Implications of $\delta_{CP}=-90^\circ$ towards determining hierarchy and octant at T2K and T2K-II