On Manin's conjecture for singular del Pezzo surfaces of degree four, I
  In this paper the height zeta function associated to a certain singular del Pezzo surface of degree four is studied. If $U$ denotes the open subset formed by deleting the unique line from this surface, then an asymptotic formula for the number of rational points of bounded height on $U$ is established which verifies the Manin conjecture. 
R. de la Breteche and T.D. Browning