A search for neutral heavy resonances is performed in the $WW\rightarrow e\nu \mu \nu $ decay channel using pp collision data corresponding to an integrated luminosity of $36.1\,\hbox {fb}^{-1}$ , collected at a centre-of-mass energy of 13 $\,\text {TeV}$ by the ATLAS detector at the Large Hadron Collider. No evidence of such heavy resonances is found. In the search for production via the quark?antiquark annihilation or gluon?gluon fusion process, upper limits on $\sigma _X\times B(X \rightarrow WW)$ as a function of the resonance mass are obtained in the mass range between 200 $\,\text {GeV}$ and up to 5 $\,\text {TeV}$ for various benchmark models: a Higgs-like scalar in different width scenarios, a two-Higgs-doublet model, a heavy vector triplet model, and a warped extra dimensions model. In the vector-boson fusion process, constraints are also obtained on these resonances, as well as on a Higgs boson in the Georgi?Machacek model and a heavy tensor particle coupling only to gauge