Variability in the cycle length of the supersoft source RX J0513.9-6951 V. Burwitz, K. Reinsch, J. Greiner, E. Meyer-Hofmeister, F. Meyer, F. M. Walter and R. E. Mennickent A&A, 481 1 (2008) 193-198 Published online: 04 February 2008 DOI: 10.1051/0004-6361:20067010