  Scattered light high-resolution imaging of the proto-planetary disc orbiting
HD100453 shows two symmetric spiral arms, possibly launched by an external
stellar companion. In this paper we present new, sensitive high-resolution
($\sim$30 mas) Band 7 ALMA observations of this source. This is the first
source where we find counterparts in the sub-mm continuum to both scattered
light spirals. The CO J=3-2 emission line also shows two spiral arms; in this
case they can be traced over a more extended radial range, indicating that the
southern spiral arm connects to the companion position. This is clear evidence
that the companion is responsible for launching the spirals. The pitch angle of
the sub-millimeter continuum spirals ($\sim 6 ^{\circ}$) is lower than the one
in scattered light ($\sim 16 ^{\circ}$). We show that hydrodynamical
simulations of binary-disc interaction can account for the difference in pitch
angle only if one takes into account that the midplane is colder than the upper
layers of the disc, as expected for the case of externally irradiated discs.
