  If neutrinos get mass through the exchange of lepton mediators, as in seesaw
schemes, the neutrino appearance probabilities in oscillation experiments are
modified due to effective nonunitarity of the lepton mixing matrix. This also
leads to new CP phases and an ambiguity in underpinning the ''conventional''
phase of the three-neutrino paradigm. We study the CP sensitivities of various
setups based at the European spallation source neutrino super-beam (ESSnuSB)
experiment in the presence of nonunitarity. We also examine its potential in
constraining the associated new physics parameters. Moreover, we show how the
combination of DUNE and ESSnuSB can help further improve the sensitivities on
the nonunitarity parameters.
