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Transmission spectroscopy is a powerful technique for probing exoplanetaryatmospheres. A successful ground-based observational method uses a differentialtechnique based on high-dispersion spectroscopy, but that only preserves narrowfeatures in transmission spectra. Here we use the chromatic Rossiter-McLaughlin(RM) effect to measure the Rayleigh-scattering slope in the transmissionspectrum of HD 189733b with the aim to show that it can be effectively used tomeasure broadband transmission features. The amplitude of the RM effectsdepends on the effective size of the planet, and in the case of an atmosphericcontribution therefore depends on the observed wavelength. We analysed archivalHARPS data of three transits of HD 189733b, covering a wavelength range of 400to 700 nm. We measured the slope in the transmission spectrum of HD 189733b ata $2.5\sigma$ significance. Assuming it is due to Rayleigh scattering and notcaused by stellar activity, it would correspond to an atmospheric temperature,as set by the scale height, of $T = 2300 \pm 900 \mathrm{K}$, well in line withpreviously obtained results. This shows that ground-based high-dispersionspectral observations can be used to probe broad-band features in thetransmission spectra of extrasolar planets, by using the chromatic RM effect.This method will be particularly interesting in conjunction with the newechelle spectrograph ESPRESSO, which currently is under construction for ESOsVery Large Telescope and will provide a gain in signal-to-noise ratio of abouta factor 4 compared to HARPS. This will be of great value because of thelimited and uncertain future of the Hubble Space Telescope and because thefuture James Webb Space Telescope will not cover this wavelength regime.

Using the chromatic Rossiter-McLaughlin effect to probe the broadband signature in the optical transmission spectrum of HD 189733b