  The Froggatt-Nielsen mechanism is a well-motivated framework for generating
the fermion mass hierarchy. This mechanism introduces flavons, complex scalars
which are singlet under the Standard Model gauge symmetry and charged under a
new global family symmetry. We make use of a leptophilic flavon to produce the
charged lepton Yukawa matrix. The real part of the flavon mixes with the Higgs
boson and introduces lepton flavour violating interactions which are bounded by
experiment. The imaginary part of the flavon, $\eta$, is a long-lived light
particle, whose abundance is restricted by cosmological observations. For
$m_\eta < 2m_e$ where the decay of $\eta$ to charged leptons is kinematically
forbidden, we identify allowed regions of $m_\eta$ with respect to the vacuum
expectation value of the flavon field where all experimental and cosmological
constraints are satisfied.
