  The observation of low-frequency gravitational waves with the Laser
Interferometer Space Antenna will allow the study of new sources of
gravitational radiation that are not accessible by ground-based instruments.
Gravitational wave sources provide invaluable information both about their
astrophysics, as well as the nature of the gravitational interaction in their
neighborhoods. One low frequency source that has not received much attention
regarding the latter are galactic binaries composed of two white dwarves or a
white dwarf and a neutron star. We here show that, contrary to the common lore,
such gravitational wave sources can indeed be used to constrain an important
feature of the gravitational interaction: the absence of pre-Newtonian, dipolar
dissipation. We propose a model-independent framework to carry out a null test
for the presence of this feature in the data that is very much analogous to
tests of General Relativity with radio-observations of binary pulsars. We then
go one step further and specialize this test to scalar-tensor theories to
derive projected constraints on spontaneous scalarization. We find that these
constraints can be comparable to current bounds with binary pulsars, and in
some optimistic cases, they can be even stronger.
