1997A&A...321..477B

We have analysed 53 optical spectra (40 taken during the outburst peak and 13 during the decline) of the Soft X-ray Transient GRO J1655-40 (=X-Ray Nova Scorpii 1994). The H_α flux during outburst shows a periodicity of 1.3-days, which is almost exactly one half of the one obtained by Bailyn et al. (1995b) from radial velocity variations. A possible explanation for this behaviour is provided. We also notice that the spectral emission features shrink or turn into absorption lines with an emission core during the decline. This is typical of Soft X-ray Transients and suggests that an expansion of the disk takes place after the outburst peak phase.