Combination of the searches for pair-produced vector-like partners of the third generation quarks at $\sqrt{s} =$~13~TeV with the ATLAS detector

A combination of the searches for pair-produced vector-like partners of the top and bottom quarks in various decay channels ($T$$\rightarrow$$Zt/Wb/Ht$, $B$$\rightarrow$$Zb/Wt/Hb$) is performed using 36.1 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the Standard Model background prediction and upper limits are set at 95\% confidence level on the production cross-section for a range of vector-like quark scenarios, significantly extending the reach of the individual searches. A singlet $T$ is excluded for masses below 1.31 TeV and a singlet $B$ is excluded for masses below 1.22 TeV. Assuming a weak isospin $(T,B)$ doublet, $T$ masses below 1.37 TeV and $B$ masses below 1.37 TeV are excluded.