  Context. The characterisation of exoplanet atmospheres has shown a wide
diversity in compositions. Hot Jupiters have the appropriate temperatures to
host metallic compounds, which should be detectable through transmission
spectroscopy. Aims. We aim to detect exotic species in transmission spectra of
hot Jupiters, specifically WASP-31b, by testing a variety of chemical species
to explain the spectrum. Methods. We conduct a re-analysis of publicly
available transmission data of WASP-31b using the Bayesian retrieval framework
TauREx II. We retrieve various combinations of the opacities of 25 species to
determine the minimum set needed to fit the observed spectrum. Results. We
report evidence for the spectroscopic signatures of chromium hydride (CrH), H2O
and K in WASP-31b. Compared to a flat model without any signatures, a CrH-only
model is preferred with a statistical significance of ~3.9 sigma. A model
consisting of both CrH+H2O is found with ~2.6 sigma and ~3 sigma confidence
over a CrH-only and H2O-only model respectively. Weak evidence for the addition
of K is found at ~2.2 sigma over the H2O+CrH model, although the fidelity of
the data point associated with this signature was questioned in earlier
studies. Finally, the inclusion of collision-induced absorption and a Rayleigh
scattering slope is found with ~3.5 sigma confidence over a flat model. This
analysis presents the first evidence for signatures of CrH in a hot Jupiter
atmosphere. At a retrieved temperature of ~1481 K, the atmosphere of WASP-31b
is hot enough to host gaseous Cr-bearing species. The retrieved abundances
agree well with predictions from thermal equilibrium chemistry and with the
abundance found in an L-type brown dwarf atmosphere. However, additional
retrievals using VLT FORS2 data lead to a non-detection of CrH. Future coverage
with JWST has the potential to confirm the detection and discover other CrH
features.
