by the *Swift* Ultra-Violet/Optical Telescope at a position R.A. $=16^\mathrm{h}45^\mathrm{m}04.01^\mathrm{s}$ and Decl. $=+13^\circ49\arcmin35.9\arcsec$ [J2000, estimated 90 per cent confidence error radius of 0.6; @Marshall08]. We retrieved the X-ray light curve and spectra from the *Swift* repository [@Evans07; @Evans09].\

ULTRASPEC imaging
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![image](fig2.eps){width="180mm"}

For 26.45 min, starting at 08:58 UT on February 10th 2008, we observed with ULTRASPEC at the ESO 3.6-m telescope on La Silla, Chile, mounted on the EFOSC2 spectrograph [@D'Odorico88]. Because of the frame-transfer capabilities of ULTRASPEC, it is possible to obtain very high time resolution data without sacrificing efficiency. Observations were taken in imaging mode, through the Bessel $V$-band filter. The CCD pixels were binned by $2\times2$, allowing $\sim$1 s sampling of the light curve, with negligible ($\sim 10$ms) dead time between exposures. Time-stamping of individual exposures uses a dedicated GPS-based system with a relative accuracy of 50 $\mu$s and an absolute accuracy of a few ms. The data were bias subtracted and subsequently flat-fielded using a median of 100 sky-flat frames.\
The photometric information about the GRB afterglow was extracted using an implementation of the optimal photometry algorithm of @Naylor98, which provides significantly better signal-to-noise than aperture photometry for faint sources. A nearby comparison star was 