We derive a general formula for the quadratic embedding constant of a graph join Km + G, where Km is the empty graph on m ≥ 1 vertices and G is an arbitrary graph. Applying our formula to a fan graph K 1 + P n , where K 1 = K1 is the singleton graph and P n is the path on n ≥ 1 vertices, we show that QEC(K 1 + P n ) = − αn − 2, where αn is the minimal zero of a new polynomial Φ n (x) related to Chebyshev polynomials of the second kind. Moreover, for an even n we have αn = min ev(A n ), where the right-hand side is the An minimal eigenvalue of the adjacency matrix A n of P n . For an odd n we show that min ev(A n+1 ) ≤ αn < min ev(A n ).