Résume | There is an arithmetic refinement of the Yau-Zaslow formula by replacing the classical Euler characteristic in Beauville's argument by a variant of Levine's motivic Euler characteristic. This result implies several similar formulas for other related invariants, including Saito's determinant of cohomology, and a generalisation of a formula of Kharlamov and Rasdeaconu on counting real rational curves on real K3 surfaces. The methods of the proof are classical, and don't use motivic homotopical tools. |