  We report on a deep spectroscopic survey for star-to-star CN variations along
the main sequence (MS) of the globular cluster 47 Tuc with ESO's VLT. We find a
significant bimodal distribution in the S(3839) index for main-sequence stars
in the mass range of ~0.85 to 0.65 M_sun, or from the main-sequence turn-off
down to ~2.5 mag below the main sequence turn-off. An anti-correlation of CN
and CH is evident on the MS. The result is discussed in the context of the
ability of faint MS stars to alter their surface composition through internal
evolutionary effects. We argue against internal stellar evolution as the only
origin for the abundance spread in 47 Tuc; an external origin such as pollution
seems to be more likely.
