  Low energy effects of generic extensions of the Standard Model can be
comprehensively parametrized in terms of higher dimensional effective
operators. After the success of all the recent precission tests on the Standard
Model, we argue that any sensible description of these extensions at the
Z-scale must be stable under higher order quantum corrections. The imposition
of $SU(2)_L \times U(1)_Y$ gauge invariance seems to be the simplest and most
natural way to fulfill this requirement. With this assumption, all the possible
deviations from the standard triple gauge boson vertices can be consistently
parametrized in terms of a finite set of gauge invariant operators. We deal
here with those operators that do not give any tree level effect on present
experimental observables and constrain them by computing their effects at the
one-loop level. We conclude that for a light Higgs boson, the direct
measurement at LEP200 can improve present bounds on these "blind directions",
while for a heavy Higgs it is most unlikely to provide any new information.
