<p>A search for the production of heavy partners of the top quark with charge <inline-formula><mml:math display="inline"><mml:mrow><mml:mn>5</mml:mn><mml:mo>/</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:math></inline-formula> is performed in events with a pair of same-sign leptons. The data sample corresponds to an integrated luminosity of <inline-formula><mml:math display="inline"><mml:mrow><mml:mn>19.5</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:msup><mml:mrow><mml:mi>fb</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula> and was collected at <inline-formula><mml:math display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:mi>s</mml:mi></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>8</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:mrow></mml:math></inline-formula> by the CMS experiment. No significant excess is observed in the data above the expected background, and the existence of top-quark partners with masses below 800 GeV is excluded at a 95% confidence level, assuming they decay exclusively to <inline-formula><mml:math display="inline"><mml:mi>t</mml:mi><mml:mi>W</mml:mi></mml:math></inline-formula>. This is the first limit on these particles from the LHC, and it is significantly more restrictive than previous limits.</p>