  FeSe is a unique member of the family of iron-based superconductors, not only
because of the high values of $T_{c}$ in FeSe monolayer, but also because in
bulk FeSe superconductivity emerges inside a nematic phase without competing
with long-range magnetic order. Near $T_{c}$, superconducting order necessarily
has $s+d$ symmetry, because nematic order couples linearly the $s$-wave and
$d$-wave harmonics of the superconducting order parameter. Here we argue that
the near-degeneracy between $s$-wave and $d$-wave pairing instabilities in
FeSe, combined with the sign-change of the nematic order parameter between hole
and electron pockets, allows the superconducting order to break time-reversal
symmetry at a temperature $T^{*}<T_{c}$. The transition from an $s+d$ state to
an $s+\mathrm{e}^{i\alpha}d$ state should give rise to a peak in the specific
heat and to the emergence of a soft collective mode that can be potentially
detected by Raman spectroscopy.
