  A search for pair production of supersymmetric particles in events with two
oppositely charged leptons (electrons or muons) and missing transverse momentum
is reported. 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 during the 2016 data taking period at the LHC. No significant
deviation is observed from the predicted standard model background. The results
are interpreted in terms of several simplified models for chargino and top
squark pair production, assuming $R$-parity conservation and with the
neutralino as the lightest supersymmetric particle. When the chargino is
assumed to undergo a cascade decay through sleptons, with a slepton mass equal
to the average of the chargino and neutralino masses, exclusion limits at 95%
confidence level are set on the masses of the chargino and neutralino up to 800
and 320 GeV, respectively. For the top squark pair production, the search
focuses on models with a small mass difference between the top squark and the
lightest neutralino. When the top squark decays into an off-shell top quark and
a neutralino, the limits extend up to 420 and 360 GeV for the top squark and
neutralino masses, respectively.
