XTE J1859+226 is a transient X-ray binary discovered in 1999 by the X-ray
satellite RXTE. This type of binary system stays in quiescence for most of its life; however, from
time-to-time the system can show an outburst at all wavelengths which can subsequently be detected by X-ray satellites. By combining the data taken using the INT and the WHT in 2000 with the
photometry taken in 2008 with NOT, WHT and spectra from GTC, a lower limit to the mass of the black hole was set to 5.42 solar masses.