We fit the 1 S 0 ΛΛ interaction in the nuclear medium to the masses of the experimentally known double−Λ hypernuclei: 6 ΛΛ He, 10 ΛΛ Be and 13 ΛΛ B. We derive this effective interaction from OBE Jülich ΛΛ-type potentials and using both HartreeFock and variational approaches. We find that the inclusion of ΛΛ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double−Λ hypernuclei to the ωΛΛ coupling and the form factor at the σΛΛ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double−Λ hypernuclei, not discovered yet. Finally, we discard the existence of 1 S 0 ΛΛ bound states provided the ΛΛ − ΞN coupling can be neglected.