  A search for Higgs boson pair production in the $b\bar{b}WW^{*}$ decay mode
is performed in the $b\bar{b}l\nu qq$ final state using 36.1 fb$^{-1}$ of
proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with
the ATLAS detector at the Large Hadron Collider. No evidence of events beyond
the background expectation is found. Upper limits on the non-resonant $pp\to
HH$ production cross section of 10 pb and on the resonant production cross
section as a function of the $HH$ invariant mass are obtained. Resonant
production limits are set for scalar and spin-2 graviton hypotheses in the mass
range 500 to 3000 GeV.
