  Performing a fit to the available experimental data, we quantify the effect
of long-distance contributions from penguin contractions in B^0 to J/psi K^0
decays. We estimate the deviation of the measured S_CP term of the
time-dependent CP asymmetry from sin(2 beta) induced by these contributions and
by the penguin operators. We find Delta S = 0.000 +- 0.012 ([-0.025,0.024] @
95% probability).
