  The spectrum of frequencies and characteristic times that compose the
ringdown phase of gravitational waves emitted by neutron stars carries
information about the matter content (the equation of state) and the underlying
theory of gravity. Typically, modified theories of gravity introduce additional
degrees of freedom/fields, such as scalars, which result in new families of
modes composing the ringdown spectrum. Simple but physically promising
candidates are scalar-tensor theories, which effectively introduce an
additional massive scalar field (i.e. an ultra-light boson) that couples
non-minimally to gravity, resulting in scalarized neutron stars. Here we
present the first calculation of the full ringdown spectrum in such theories.
We show that the ringdown spectrum of neutron stars with ultra-light bosons is
much richer and fundamentally different from the spectrum in general relativity
and that it possesses propagating ultra long lived modes.
