  The effect of a BULK marginal operator on BOUNDARY critical phenomena in two
space-time dimensions is considered. The particular case of an open S=1/2
antiferromagnetic Heisenberg chain, corresponding to a Wess-Zumino-Witten
non-linear sigma model, is solved. In this case, the needed renormalization
group coefficient is associated with a novel operator product expansion in
which
  THREE operators approach the same point. Resulting logarithmic corrections
occurring in finite size calculations and nuclear magnetic resonance
experiments are discussed.
