  A search for direct production of the supersymmetric (SUSY) partners of
electrons or muons is presented in final states with two opposite-charge,
same-flavor leptons (electrons and muons), no jets, and large missing
transverse momentum. The data sample corresponds to an integrated luminosity of
35.9 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} =$ 13 TeV, collected
with the CMS detector at the LHC in 2016. The search uses the $m_{\mathrm{T}2}$
variable, which generalizes the transverse mass for systems with two invisible
objects and provides a discrimination against standard model backgrounds
containing W bosons. The observed yields are consistent with the expectations
from the standard model. The search is interpreted in the context of simplified
SUSY models and probes slepton masses up to approximately 290, 400, and 450
GeV, assuming right-handed only, left-handed only, and both right- and
left-handed sleptons (mass degenerate selectrons and smuons), and a massless
lightest supersymmetric particle. Limits are also set on selectrons and smuons
separately. These limits show an improvement on the existing limits of
approximately 150 GeV.
