  We prove an arithmetic refinement of the Yau-Zaslow formula by replacing the
classical Euler characteristic in Beauville's argument by a "motivic Euler
characteristic", related to the work of Levine. Our result implies similar
formulas for other related invariants, including a generalisation of a formula
of Kharlamov and Rasdeaconu on counting real rational curves on real K3
surfaces, and Saito's determinant of cohomology.

Algebraic Geometry;Number Theory