Asai, K., Dotani, T., Mitsuda, K., Nagase, F., Kamado, Y., Kuulkers, E., & Breedon, L. M. (1994). Spectral variations along the branches in GX 5-1. Publications of the Astronomical Society of Japan, 46, 479-491.
Kuulkers, E., & van der Klis, M. (1993). G 5-1, or: how to become a z source. In O. Regev, & G. Shaviv (Eds.), Cataclysmic variables and related physics: 2nd Technion Haifa Conference (pp. 296). (Annals of the Israel Physical Society). Institute of Physics Publishing.