  In this note, which is an addendum to the e-print math.AG/9810121, we prove
that the variety VSP(F,10) of presentations of a general cubic form F in 6
variables as a sum of 10 cubes is a smooth symplectic 4-fold, which is
deformation equivalent to the Hilbert square of a K3 surface of genus 8 but
different from the family of lines on a cubic 4-fold. This provides a new
geometric construction of a compact complex symplectic fourfold, different from
a Hilbert square of a K3 surface, a generalized Kummer 4-fold, the variety of
lines on a cubic 4-fold and the recent examples of O'Grady (see Duke Math. J.
134, no. 1 (2006), 99-137).
