  A BRST perturbative analysis of SU(N) Yang-Mills theory in a class of maximal
Abelian gauges is presented. We point out the existence of a new nonintegrated
renormalizable Ward identity which allows to control the dependence of the
theory from the diagonal ghosts. This identity, called the diagonal ghost
equation, plays a crucial role for the stability of the model under radiative
corrections implying, in particular, the vanishing of the anomalous dimension
of the diagonal ghosts. Moreover, the Ward identity corresponding to the
Abelian Cartan subgroup is easily derived from the diagonal ghost equation.
Finally, a simple proof of the fact that the beta function of the gauge
coupling can be obtained from the vacuum polarization tensor with diagonal
gauge fields as external legs is given. A possible mechanism for the decoupling
of the diagonal ghosts at low energy is also suggested.
