Let SQ denote the cubic hypersurface
x3 = Q(y1, ..., ym)z,
where Q is a positive definite quadratic form in m variables
with integer coefficients. This SQ ranges over a class of singular cubic
hypersurfaces as Q varies. For SQ, we prove:
(i) Manin's conjecture is true if Q is locally determined, and we give an explicit asymptotic
formula with a power
saving error term;
(ii) in general Manin's conjecture is true up to a leading constant if m ≥ 6 is even.