  We show that the X-ray source W31 in the core of the globular cluster 47
Tucanae is physically associated with the bright blue straggler BSS-7. The two
sources are astrometrically matched to 0.061\arcsec, with a chance coincidence
probability of less than 1%. We then analyse optical time-series photometry
obtained with the {\em Hubble Space Telescope} (HST) and find that BSS-7
displays a 1.56 day periodic signal in the I band. We also construct a
broad-band (far-ultraviolet through far-red) spectral energy distribution for
BSS-7 and fit this with single and binary models. The binary model is a better
fit to the data, and we derive the corresponding stellar parameters.
  All of our findings are consistent with BSS-7 being a detached binary
consisting of a blue straggler primary with an X-ray-active,
upper-main-sequence companion. The formation of such a system would necessarily
involve at least three stars, which is consistent with recent N-body models in
which blue stragglers often form via multiple encounters that can involve both
single and binary stars. However, we cannot yet entirely rule out the
possibility that BSS-7 descended directly from a binary system via mass
transfer. The system parameters needed to distinguish definitively between
these scenarios may be obtainable from time-resolved spectroscopy.
