Invariants of Vanishing Brauer Classes

A specialization of a K3 surface with Picard rank one to a K3 with rank two de nes a vanishing class of order two in the Brauer group of the general K3 surface. We give the B- eld invariants of this class. We apply this to the K3 double plane de ned by a cubic fourfold with a plane. The specialization of such a cubic fourfold whose group of codimension two cycles has rank two to one which has rank three induces such a specialization of the double planes. We determine the Picard lattice of the specialized double plane as well as the vanishing Brauer class and its relation to the natural `Cli ord' Brauer class. This provides more insight in the specializations. It allows us to explicitly determine the K3 surfaces associated to in nitely many of the conjec- turally rational cubic fourfolds obtained as such specializations.