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We present high-time resolution optical spectroscopy and imaging of the blackhole transient Swift J1357.2-0933 during its 2017 outburst. The light curvesshow recurrent dips resembling those discovered during the 2011 outburst. Thedip properties (e.g duration and depth) as well as the evolution of theirrecurrence time are similar to those seen in 2011. Spectra obtained during thedips are characterised by broad and blue-shifted absorptions in Balmer and Heii. The absorptions show core velocities of $\sim$ -800 km/s and terminalvelocities approaching $\sim$ 3000 km/s i.e. in the upper-end of windvelocities measured in other black hole transients (both at optical and X-raywavelengths). Our observations suggest that the dips are formed in a dense andclumpy outflow, produced near the disc equatorial plane and seen at highinclination. We also study the colour evolution and observe that, as it hasbeen previously reported, the source turns bluer during dips. We show that thisis due to a gradual change in the slope of the optical continuum and discusspossible implications of this behaviour.

An equatorial outflow in the black hole optical dipper Swift J1357.2-0933