  We construct a chiral Lagrangian for doubly heavy baryons and heavy mesons
that is invariant under heavy quark-diquark symmetry at leading order and
includes the leading O(1/m_Q) symmetry violating operators. The theory is used
to predict the electromagnetic decay width of the J=3/2 member of the ground
state doubly heavy baryon doublet. Numerical estimates are provided for doubly
charm baryons. We also calculate chiral corrections to doubly heavy baryon
masses and strong decay widths of low lying excited doubly heavy baryons.
