  The critical behaviour of many spin models can be equivalently formulated as
percolation of specific site-bond clusters. In the presence of an external
magnetic field, such clusters remain well-defined and lead to a percolation
transition, even though the system no longer shows thermal critical behaviour.
We investigate the 2-dimensional Ising model and the 3-dimensional O(2) model
by means of Monte Carlo simulations. We find for small fields that the line of
percolation critical points has the same functional form as the line of thermal
pseudocritical points.
