  A search is presented for supersymmetric partners of the top quark (top
squarks) in final states with two oppositely charged leptons (electrons or
muons), jets identified as originating from b quarks, and missing transverse
momentum. The search uses data from proton-proton collisions at $\sqrt{s} =$ 13
TeV collected with the CMS detector, corresponding to an integrated luminosity
of 137 fb$^{-1}$. Hypothetical signal events are efficiently separated from the
dominant top quark pair production background with requirements on the
significance of the missing transverse momentum and on transverse mass
variables. No significant deviation is observed from the expected background.
Exclusion limits are set in the context of simplified supersymmetric models
with pair-produced lightest top squarks. For top squarks decaying exclusively
to a top quark and a lightest neutralino, lower limits are placed at 95%
confidence level on the masses of the top squark and the neutralino up to 925
and 450 GeV, respectively. If the decay proceeds via an intermediate chargino,
the corresponding lower limits on the mass of the lightest top squark are set
up to 850 GeV for neutralino masses below 420 GeV. For top squarks undergoing a
cascade decay through charginos and sleptons, the mass limits reach up to 1.4
TeV and 900 GeV respectively for the top squark and the lightest neutralino.
