  A search for Higgs boson production in association with a $W$ or $Z$ boson,
in the $H \rightarrow WW^{\ast}$ decay channel, is performed with a data sample
collected with the ATLAS detector at the LHC in proton-proton collisions at
centre-of-mass energies $\sqrt{s}=7$ TeV and 8 TeV, corresponding to integrated
luminosities of 4.5 ${\rm fb}^{-1}$ and 20.3 ${\rm fb}^{-1}$, respectively. The
$WH$ production mode is studied in two-lepton and three-lepton final states,
while two-lepton and four-lepton final states are used to search for the $ZH$
production mode. The observed significance, for the combined $WH$ and $ZH$
production, is 2.5 standard deviations while a significance of 0.9 standard
deviations is expected in the Standard Model Higgs boson hypothesis. The ratio
of the combined $WH$ and $ZH$ signal yield to the Standard Model expectation,
$\mu_{VH}$, is found to be $\mu_{VH} =3.0^{+1.3}_{-1.1}{\, {(\rm
stat.)}}^{+1.0}_{-0.7}{\,{(\rm sys.)}}$ for the Higgs boson mass of 125.36 GeV.
The $WH$ and $ZH$ production modes are also combined with the gluon fusion and
vector boson fusion production modes studied in the $H \to WW^* \to \ell \nu
\ell \nu$ decay channel, resulting in an overall observed significance of 6.5
standard deviations and $\mu_{\rm ggF+VBF+VH} = 1.16^{+0.16}_{-0.15} {\rm
(stat.)}^{+0.18}_{-0.15} {\rm (sys.)}$. The results are interpreted in terms of
scaling factors of the Higgs boson couplings to vector bosons ($\kappa_V$) and
fermions ($\kappa_F$); the combined results are: $|\kappa_V| =
1.06^{+0.10}_{-0.10}$, $|\kappa_F| = 0.85^{+0.26}_{-0.20}$.
