  We report on the results of a campaign to observe the SXT GRO J1655-40 in
outburst using HST, XTE and GRO and AAT. This outburst was qualitatively quite
different to other SXT outbursts and to previous outbursts of this source. The
onset of hard X-ray activity occurred very slowly, over several months and was
delayed relative to the soft X-ray rise. During this period, the optical fluxes
declined steadily. This apparent anti-correlation is not consistent with the
standard disc instability model of SXT outbursts, nor is it expected if the
optical output is dominated by reprocessing. Using our derived reddening we
find that our dereddened spectra are dominated by a component peaking in the
optical with the expected nu^1/3 disc spectrum seen only in the UV. We consider
possible interpretations of this spectrum in terms of thermal emission from the
outer accretion disc and/or secondary star, both with and without X-ray
irradiation, and also as non-thermal optical synchrotron emission from a
compact self-absorbed central source. The X-ray spectra broadly resemble the
high-soft state commonly seen in black hole candidates, but evolve through two
substates. The outburst dataset cannot readily be interpreted by any standard
model for SXT outbursts. Many of the characteristics could be interpreted with
a model combining X-ray irradiation with the disc instability, but with the
added ingredient of a very large disc in this long period system.
