Optical observations of the black hole candidate XTE J1550-564 during the September/October 1998 outburst

We report here optical observations during the September/October 1998 outbu rst of the black hole candidate XTE J1550-564. CCD photometry was obtained for the optical counterpart with the 0.9m Dutch telescope at La Silla since the onset of this event. We analysed 211 U, V and i frames, from Sep 10 to Oct 23. Stochastic flaring activity was observed on Sep 11-16, but no evid ence for a superhump period, as seen in other soft X-ray transients, was fo und. An optical flare nearly simultaneous to an X-ray flare that occurred o n Sep 21 was observed in the V-band. A reddened optical spectrum showed the typical emission lines corresponding to X-ray transients in outburst. From the interstellar absorption lines, we derive E(B-V) = 0.70 +/- 0.10 and su ggest D approximate to 2.5 kpc. In such case, M-B approximate to +7 mag. fo r the progenitor, which is consistent with the spectral type of a low-mass similar to K0-K5 main-sequence companion.