  We systematically study the $S$-wave doubly charmed baryons using the method
of QCD sum rules. Our results suggest that the $\Xi_{cc}^{++}$ recently
observed by LHCb can be well identified as the $S$-wave $\Xi_{cc}$ state of
$J^P = 1/2^+$. We study its relevant $\Omega_{cc}$ state, whose mass is
predicted to be around 3.7 GeV. We also systematically study the $P$-wave
doubly charmed baryons, whose masses are predicted to be around 4.1 GeV.
Especially, there can be several excited doubly charmed baryons in this energy
region, and we suggest to search for them in order to study the fine structure
of the strong interaction.
