  We present the results of our long-term photometric and spectroscopic
observations at the Russian-Turkish RTT-150 telescope for the optical
counterpart to one of the best-known sources, representatives of the class of
fast X-ray transients, IGR J17544-2619. Based on our optical data, we have
determined for the first time the orbital and physical parameters of the binary
system by the methods of Doppler spectroscopy. We have computed theoretical
spectra of the optical counterpart by applying non-LTE corrections for selected
lines and obtained the parameters of the stellar atmosphere and the optical
star ($T_{\rm eff}=33\,000$ K, $\log\,g=3.85$, $R=9.5\ R_{\odot},$ and $M=23\
M_{\odot}$). The latter suggest that the optical star is not a supergiant as
has been thought previously.
