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We present a 30-day monitoring campaign of the optical counterpart of thebright X-ray transient Swift J1745-26, starting only 19 minutes after thediscovery of the source. We observe the system peaking at i' ~17.6 on day 6(MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the opticalpeak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak.Our measurements result in an outburst amplitude greater than 4.3 magnitudes,which favours an orbital period < 21 h and a companion star with a spectraltype later than ~ A0. Spectroscopic observations taken with the GTC-10.4 mtelescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emissionline from which we constrain the radial velocity semi-amplitude of the donor tobe K_2 > 250 km/s. The breadth of the line and the observed optical and X-rayfluxes suggest that Swift J1745-26 is a new black hole candidate located closerthan ~7 kpc.

The optical counterpart of the bright X-ray transient Swift J1745-26