  Recently LHCb discovered the first doubly-charmed baryon $\Xi_{cc}^{++} =
ccu$ at $3621.40 \pm 0.78$ MeV, very close to our theoretical prediction. We
use the same methods to predict a doubly-bottom tetraquark $T(bb\bar u\bar d)$
with $J^P{=}1^+$ at $10,389\pm 12$ MeV, 215 MeV below the $B^-\bar B^{*0}$
threshold and 170 MeV below threshold for decay to $B^-\bar B^0 \gamma$. The
$T(bb\bar u\bar d)$ is therefore stable under strong and electromagnetic (EM)
interactions and can only decay weakly, the first exotic hadron with such a
property. On the other hand, the mass of $T(cc\bar u\bar d)$ with $J^P{=}1^+$
is predicted to be $3882\pm12$ MeV, 7 MeV above the $D^0 D^{*+}$ threshold and
148 MeV above $D^0 D^+ \gamma$ threshold. $T(bc\bar u\bar d)$ with $J^P{=}0^+$
is predicted at $7134\pm13$ MeV, 11 MeV below the $\bar B^0 D^0$ threshold. Our
precision is not sufficient to determine whether $bc\bar u\bar d\,$ is actually
above or below the threshold. It could manifest itself as a narrow resonance
just at threshold.
