  A search is presented for excited electrons and muons in $\ell\ell\gamma$
final states at the LHC. The search is based on a data sample corresponding to
an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a
center-of-mass energy of 13 TeV, collected with the CMS detector in 2016. This
is the first search for excited leptons at $\sqrt{s} =$ 13 TeV. The observation
is consistent with the standard model background prediction, and the most
stringent exclusion limits to date are set on the excited lepton mass and the
compositeness scale, at 95% confidence level. Excited electrons and muons are
excluded for masses below 3.9 and 3.8 TeV, respectively, under the assumption
that the excited lepton mass equals the compositeness scale. The best observed
limit on the compositeness scale is obtained with an excited lepton mass of
around 1.0 TeV, excluding values below 25 TeV for both excited electrons and
muons.
