We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 ( V406 Vul ) .
Photometric observations taken during 2000 and 2008 reveals the presence of the secondary star ’ s ellipsoidal modulation .
Further photometry obtained in 2010 shows the system \simeq 1 mag brighter than its quiescence level and the ellipsoidal modulation diluted by strong flaring activity .
Spectroscopic data obtained with the 10.4-m GTC in 2010 reveals radial velocity variations of \sim 500 km s ^ { -1 } over 3 h. A simultaneous fit to the photometry and spectroscopy using sinusoids to represent the secondary star ’ s ellipsoidal and radial velocity variations , yields an orbital period of 6.58 \pm 0.05 h and a secondary star ’ s radial velocity semi-amplitude of K _ { 2 } = 541 \pm 70 ~ { } km s ^ { -1 } .
The implied mass function is f ( M ) = 4.5 \pm 0.6 ~ { } M _ { \odot } , significantly lower than previously reported but consistent with the presence of a black hole in XTE J1859+226 .
The lack of eclipses sets an upper limit to the inclination of 70 degrees which yields a lower limit to the black hole mass of 5.42 M _ { \odot } .