  We present a general field-theoretic strategy to analyze three connected
families of continuous phase transitions which occur in nonequilibrium
steady-states. We focus on transitions taking place between an active state and
one absorbing state, when there exist an infinite number of such absorbing
states. In such transitions the order parameter is coupled to an auxiliary
field. Three situations arise according to whether the auxiliary field is
diffusive and conserved, static and conserved, or finally static and not
conserved.
