• accretion, accretion discs;
  • binaries: close;
  • stars: individual: XTE J1859+226 (=V406 Vul);
  • X-rays: binaries


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 reveal 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 reveal radial velocity variations of ∼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 ± 0.05 h and a secondary star's radial velocity semi-amplitude of K2= 541 ± 70 km s−1. The implied mass function is f(M) = 4.5 ± 0.6 M, 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° which yields a lower limit to the black hole mass of 5.42 M.