  We find that sub-GeV neutrino portal bosons that carry lepton number can
condense inside a proto-neutron star (newly born neutron star). These bosons
are produced copiously and form a Bose-Einstein condensate for a range of as
yet unconstrained coupling strengths to neutrinos. The condensate is a lepton
number superfluid with transport properties that differ dramatically from those
encountered in ordinary dense baryonic matter. We discuss how this phase could
alter the evolution of proto-neutron stars and comment on the implications for
neutrino signals and nucleosynthesis.
