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We report Gemini-South GMOS observations of the exoplanet system WASP-29during primary transit as a test case for differential spectrophotometry. Weuse the multi-object spectrograph to observe the target star and a comparisonstar simultaneously to produce multiple light curves at varying wavelengths.The 'white' light curve and fifteen 'spectral' light curves are analysed torefine the system parameters and produce a transmission spectrum from 515 to720nm. All light curves exhibit time-correlated noise, which we model using avariety of techniques. These include a simple noise rescaling, a Gaussianprocess model, and a wavelet based method. These methods all produce consistentresults, although with different uncertainties. The precision of thetransmission spectrum is improved by subtracting a common signal from all thespectral light curves, reaching a typical precision of ~1x10^-4 in transitdepth. The transmission spectrum is free of spectral features, and given thenon-detection of a pressure broadened Na feature, we can rule out the presenceof a Na rich atmosphere free of clouds or hazes, although we cannot rule out anarrow Na core. This indicates that Na is not present in the atmosphere, and/orthat clouds/hazes play a significant role in the atmosphere and mask the broadwings of the Na feature, although the former is a more likely explanation givenWASP-29b's equilibrium temperature of ~970 K, at which Na can form variouscompounds. We also briefly discuss the use of Gaussian process and waveletmethods to account for time correlated noise in transit light curves.

A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720nm