The August 1993 outburst of the black hole candidate GRO J0422+32

One year after the X-ray outburst, the optical counterpart of the black hole candidate GRO J0422+32 underwent an unexpected one-month outburst beginning on 10 August 1993. CCD photometry was obtained for the optical counterpart with the 1.5-m telescope at Calar Alto on 6 consecutive nigths since the onset of this event, never seen before in other transients belonging to the black-hole class. Two spectra were also taken with the 2.2-m telescope on 14 August. We suggest the Mass Transfer Instability model as possible explanation of the event on the basis of the observational facts presented here.