  Scattering of two Kerr Black Holes emitting gravitational waves can be
captured by an effective theory of a massive higher-spin field interacting with
the gravitational field. While other compact objects should activate a
multitude of non-minimal interactions it is the black holes that should be
captured by the simplest minimal interaction. Implementing massive higher-spin
symmetry via a string-inspired BRST approach we construct an action that
reproduces the correct cubic amplitude of Arkani-Hamed--Huang--Huang. The same
is achieved for the root-Kerr theory, i.e. for the minimal electromagnetic
interaction of a massive higher-spin field.
