  Extreme Mass Ratio Inspirals (EMRIs) are key sources for the future
space-based gravitational wave detector LISA, and are considered promising
probes of fundamental physics. Here, we present the first complete Bayesian
analysis of EMRI signals in theories with an additional massless scalar, which
could arise in an extension of General Relativity or of the Standard Model of
Particle Physics. We develop a waveform model accurate at adiabatic order for
equatorial eccentric orbits around spinning black holes. Using full Bayesian
inference, we forecast LISA's ability to probe the presence of new fundamental
fields with EMRI observations.
