  We report on a search for differential shifts between electronic and vibronic
transitions in carbon-rich white dwarfs BPM 27606 and Procyon B. The absence of
differential shifts within the spectral resolution and taking into account
systematic effects such as space motion and pressure shifts allows us to set
the first upper bound of astrophysical origin on the coupling between the Higgs
field and the Kreschmann curvature invariant. Our analysis provides the basis
for a more general methodology to derive bounds to the coupling of long-range
scalar fields to curvature invariants in an astrophysical setting complementary
to the ones available from high-energy physics or table-top experiments.
