  A search for the pair production of the lightest supersymmetric partner of
the top quark ($\widetilde{\mathrm{t}}_1$) is presented. The search focuses on
a compressed scenario where the mass difference between the top squark and the
lightest supersymmetric particle, often considered to be the lightest
neutralino ($\widetilde{\chi}^0_1$), is smaller than the mass of the W boson.
The proton-proton collision data were recorded by the CMS experiment at a
centre-of-mass energy of 13 TeV, and correspond to an integrated luminosity of
35.9 fb$^{-1}$. In this search, two decay modes of the top squark are
considered: a four-body decay into a bottom quark, two additional fermions, and
a $\widetilde{\chi}^0_1$; and a decay via an intermediate chargino. Events are
selected using the presence of a high-momentum jet, significant missing
transverse momentum, and a low transverse momentum electron or muon. Two
analysis techniques are used, targeting different decay modes of the
$\widetilde{\mathrm{t}}_1$: a sequential selection and a multivariate
technique. No evidence for the production of top squarks is found, and mass
limits at 95% confidence level are set that reach up to 560 GeV, depending on
the $m(\widetilde{\mathrm{t}}_1) - m(\widetilde{\chi}^0_1)$ mass difference and
the decay mode.
