  Scalar-tensor theories are among the most promising alternatives to general
relativity that have been developed to account for some long standing issues in
our understanding of gravity. Some of these theories predict the existence of a
non-linear phenomenon, spontaneous scalarisation, which can lead to the
appearance of sizeable modifications to general relativity in the presence of
compact matter distributions, namely neutron stars. On the one hand, one of the
effects of the scalar field is to modify the emission of gravitational waves,
both due to variations in the quadrupolar deformation of the star and to the
presence of additional modes of emission. On the other hand, neutron stars are
known to harbour extremely powerful magnetic fields which can affect their
structure and shape, leading in turn to the emission of gravitational waves,
this time due to a magnetic quadrupolar deformation. We investigate here how
the presence of spontaneous scalarisation can affect the magnetic deformation
of neutron stars and their emission of quadrupolar gravitational waves, both of
tensor and scalar nature. We will show that it is possible to provide simple
parameterisations of the magnetic deformation and gravitational wave power of
neutron stars in terms of their baryonic mass, circumferential radius and
scalar charge, and that a universal scaling exists independently of the
magnetic field geometry and of the parameters of the scalar-tensor theory.
Finally, we comment on the observability of the deviations of the gravitational
waves strain from general relativity by current and future observatories.
