A long-term optical and X-ray ephemeris of the polar EK Ursae MajorisK. Beuermann, J. Diese, S. Paik, A. Ploch, J. Zachmann, A. D. Schwope and F. V. HessmanA&A, 507 1 (2009) 385-388DOI: https://doi.org/10.1051/0004-6361/200912771
