Hatzidimitriou, D., Pietsch, W., Misanovic, Z., Reig, P., & Haberl, F. (2006). Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M 31 and M 33. Astronomy & Astrophysics, 451(3), 835-843. doi:10.1051/0004-6361:20054402.