  Results of a search for supersymmetry are presented using events with a
photon, an electron or muon, and large missing transverse momentum. The
analysis is based on a data sample corresponding to an integrated luminosity of
35.9 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} =$ 13 TeV, produced by
the LHC and collected with the CMS detector in 2016. Theoretical models with
gauge-mediated supersymmetry breaking predict events with photons in the final
state, as well as electroweak gauge bosons decaying to leptons. Searches for
events with a photon, a lepton, and missing transverse momentum are sensitive
probes of these models. No excess of events is observed beyond expectations
from standard model processes. The results of the search are interpreted in the
context of simplified models inspired by gauge-mediated supersymmetry breaking.
These models are used to derive upper limits on the production cross sections
and set lower bounds on masses of supersymmetric particles. Gaugino masses
below 930 GeV are excluded at the 95% confidence level in a simplified model
with electroweak production of a neutralino and chargino. For simplified models
of gluino and squark pair production, gluino masses up to 1.75 TeV and squark
masses up to 1.43 TeV are excluded at the 95% confidence level.
