Seidel-equivalence of strongly regular graphs

Seidel-equivalence of two strongly regular graphs G and G∗ with the same parameters is defined. Necessary conditions for the existence of partially balanced incomplete block designs with L 2 (4) and pseudo-T(8) association schemes are also obtained. The paper also contains the uniqueness of a certain association scheme with 16 vertices and the non-existence of another scheme with 28 vertices.