  Ultralight scalars, which are states that are either exactly massless or much
lighter than any other massive particle in the model, appear in many new
physics scenarios. Axions and majorons constitute well-motivated examples of
this type of particle. In this work, we explore the phenomenology of these
states in low-energy leptonic observables adopting a model independent approach
that includes both scalar and pseudoscalar interactions. Then, we consider
processes in which the ultralight scalar $\phi$ is directly produced, such as
$\mu \to e \, \phi$, or acts as a mediator, as in $\tau \to \mu \mu \mu$.
Finally, contributions to the charged leptons magnetic and electric moments are
studied as well. In particular, it is shown that the muon $g-2$ anomaly can be
explained provided a mechanism for suppressing the experimental bounds on the
coupling between the ultralight scalar and a pair of muons is introduced.
