  We present a ground based optical transmission spectrum of the inflated
sub-Jupiter mass planet WASP-6b. The spectrum was measured in twenty spectral
channels from 480 nm to 860nm using a series of 91 spectra over a complete
transit event. The observations were carried out using multi-object
differential spectrophotometry with the IMACS spectrograph on the Baade
telescope at Las Campanas Observatory. We model systematic effects on the
observed light curves using principal component analysis on the comparison
stars, and allow for the presence of short and long memory correlation
structure in our Monte Carlo Markov Chain analysis of the transit light curves
for WASP-6. The measured transmission spectrum presents a general trend of
decreasing apparent planetary size with wavelength and lacks evidence for broad
spectral features of Na and K predicted by clear atmosphere models. The
spectrum is consistent with that expected for scattering that is more efficient
in the blue, as could be caused by hazes or condensates in the atmosphere of
WASP-6b. WASP-6b therefore appears to be yet another massive exoplanet with
evidence for a mostly featureless transmission spectrum, underscoring the
importance that hazes and condensates can have in determining the transmission
spectra of exoplanets.
