  The decay B0 -> J/psi K0_S is reconstructed with J/psi -> e+ e- or mu+ mu-
and K0_S -> pi+ pi-. From the full ALEPH dataset at LEP1 of about 4 million
hadronic Z decays, 23 candidates are selected with an estimated purity of 71%.
They are used to measure the CP asymmetry of this decay, given by sin 2beta in
the Standard Model, with the result sin 2beta = 0.84 +0.82-1.04 +-0.16. This is
combined with existing measurements from other experiments, and increases the
confidence level that CP violation has been observed in this channel to 98%.
