Covariant Insertions of General Vertex Operators

We provide a very simple prescription for inserting an arbitrary state into a string amplitude. The corresponding string measure is defined without any additional information whenever the state is Becchi-Rouet-Stora-Tyutin (BRST) invariant, generalizing the usual physical conditions. In particular no world-sheet metric is needed. We recover and interpret in a simple way the b^ prescription of Polchinski and explain geometrically how it secures the decoupling of BRST spurious states.