  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 ~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 ~500 km/s 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+-0.05 h and a secondary star's radial velocity semi-amplitude of
K_2= 541+-70 km/s. The implied mass function is f(M)=4.5+-0.6 Msun,
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 Msun.
