  We recall the main features of metric vacuum fluctuations which have the
global property int d^4x sqrt{g(x)} R(x) = 0, even though R(x) does not vanish
locally. We stress that these fluctuations could mediate an anomalous coupling
between the gravitational field and coherent matter. Some new issues are
discussed: (1) these fluctuations still imply that <T_{mu nu}(x)>=0; (2) they
are not extrema of the action; (3) for finite duration, their volume in phase
space is not zero; (4) vacuum fluctuations of this kind are not allowed in QED;
(5) their null-action property is a nonperturbative feature; (6) any -real-
pure e.m. field generates zero-modes of this kind, too, up to terms of order
G^2.
