  We report the identification and study of an unusual soft state of the black
hole low-mass X-ray binary GRO J1655-40, observed during its 2005 outburst by
the Rossi X-ray Timing Explorer. Chandra X-ray grating observations have
revealed a high mass-outflow accretion disc wind in this state, and we show
that the broadband X-ray spectrum is remarkably similar to that observed in the
so-called `hypersoft' state of the high mass X-ray binary Cyg X-3, which
possesses a strong stellar wind from a Wolf-Rayet secondary. The power-spectral
density (PSD) of GRO J1655-40 shows a bending power-law shape, similar to that
of canonical soft states albeit with larger fractional rms. However, the
characteristic bend-frequency of the PSD is strongly correlated with the X-ray
flux, such that the bend-frequency increases by two decades for less than a
factor 2 increase in flux. The strong evolution of PSD bend-frequency for very
little change in flux or X-ray spectral shape seems to rule out the suppression
of high-frequency variability by scattering in the wind as the origin of the
PSD bend. Instead, we suggest that the PSD shape is intrinsic to the
variability process and may be linked to the evolution of the scale-height in a
slim disc. An alternative possibility is that variability is introduced by
variable absorption and scattering in the wind. We further argue that the
hypersoft state in GRO J1655-40 and Cyg X-3 is associated with accretion close
to or above the Eddington limit.
