Graphs cospectral with NU(n + 1,q2), n ≠ 3

Abstract Let H ( n , q 2 ) be a non–degenerate Hermitian variety of PG ( n , q 2 ) , n ≥ 2 . Let NU ( n + 1 , q 2 ) be the graph whose vertices are the points of PG ( n , q 2 ) ∖ H ( n , q 2 ) and two vertices P 1 , P 2 are adjacent if the line joining P 1 and P 2 is tangent to H ( n , q 2 ) . Then NU ( n + 1 , q 2 ) is a strongly regular graph. In this paper we show that NU ( n + 1 , q 2 ) , n ≠ 3 , is not determined by its spectrum.