  We consider the approximation of singularly perturbed linear second-order
boundary value problems by $hp$-finite element methods. In particular, we
include the case where the associated differential operator may not be
coercive. Within this setting we derive an a posteriori error estimate for a
natural residual norm. The error bound is robust with respect to the
perturbation parameter and fully explicit with respect to both the local mesh
size $h$ and the polynomial degree $p$.
