Relativistic corrections to the hyperfine splitting are calculated for the triplet 2 3 S 1 state of the helium isotope He 3 . High precision variational wave functions are employed, where the electron-electron correlations are well accounted for. Due to the unknown nuclear structure a comparison with the experimental result is performed via He 3+ hyperfine splitting. Surprisingly, the second order contribution due to Fermi interaction, in spite of the additional small factor m/M , is very significant.
PACS numbers 31