  We present two contiguous nights of simultaneous time-resolved GTC
spectroscopy and WHT photometry of the black hole X-ray transient XTE
J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the
individual spectra against a late K-type spectral template enabled us to
constrain the orbital period to $0.276 \pm 0.003$ d and the radial velocity
semi-amplitude of the donor star to $K_2 = 550 \pm 59$ km s$^{-1}$. An
ellipsoidal modulation is detected in the photometric $r$- and $i$-band light
curves, although it is strongly contaminated by flickering activity. By
exploiting correlations between the properties of the double-peaked H$\alpha$
emission-line profile and the binary parameters, we derived an orbital
inclination of $66.6 \pm 4.3$ deg, a refined $K_2 = 562 \pm 40$ km s$^{-1}$ and
mass ratio $q = M_2/M_1 = 0.07 \pm 0.01$. From these values we obtained an
updated black hole mass of $M_1 = 7.8 \pm 1.9$ M$_\odot$. An independent mass
estimate based on X-ray timing agrees well with our value, which gives further
support for the outburst QPO triplet being explained by the relativistic
precession model. We also obtained a companion star mass $M_2 = 0.55 \pm 0.16$
M$_\odot$, which is consistent with its K5-K7 V spectral type.
