Invariants of the heat equation for non-minimal operators

A special class of non-minimal operators which are relevant for quantum field theory is introduced. The general form of the heat kernel coefficients of these operators on manifolds without boundary is described. New results are presented for the traces of the first two heat kernel coefficients for vector, Yang-Mills and perturbative gravity. It is argued that non-minimal operators can be used to define gauge-fixing independent actions and solve the conformal mode problem in quantum gravity.