  In the context of the adaptive finite element method (FEM), ZZ-error
estimators named after Zienkiewicz and Zhu are mathematically well-established
and widely used in practice. In this work, we propose and analyze ZZ-type error
estimators for the adaptive boundary element method (BEM). We consider
weakly-singular and hyper-singular integral equations and prove, in particular,
convergence of the related adaptive mesh-refining algorithms.
