  We have obtained optical spectra of the soft X-ray transient GRO J1655-40
during different X-ray spectral states (quiescence, high-soft and hard
outburst) between 1994 Aug and 1997 Jun. Characteristic features observed
during the 1996-97 high-soft state were: a) broad absorption lines at Halpha
and Hbeta, probably formed in the inner disk; b) double-peaked HeII 4686
emission lines, formed in a temperature-inversion layer on the disk surface,
created by the soft X-ray irradiation; c) double-peaked Halpha emission, with a
strength associated with the hard X-ray flux, suggesting that it was probably
emitted from deeper layers than He II 4686. The observed rotational velocities
of all the double-peaked lines suggest that the disk was extended slightly
beyond its tidal radius. Three classes of lines were identified in the spectra
taken in 1994 Aug-Sep, during a period of low X-ray activity between two strong
X-ray flares: broad absorption, broad (flat-topped) emission and narrow
emission. We have found that the narrow emission lines (single-peaked or
double-peaked) cannot be explained by a thin-disk model. We propose that the
system was in a transient state, in which the accretion disk might have had an
extended optically thin cocoon and significant matter outflow. After the onset
of a hard X-ray flare the disk signatures disappeared, and strong single-peaked
Halpha and Paschen emission was detected, suggesting that the cocoon became
opaque to optical radiation. High-ionisation lines disappeared or weakened. Two
weeks after the end of the flare, the cocoon appeared to be once again
optically thin.
