Variability in the cycle length of the supersoft source RX J0513.9-6951

Context.The supersoft X-ray binary RX J0513.9-6951 shows cyclic changes between optical-low/X-ray-on states and optical-high/X-ray-off states. It is supposed to be accreting close to the Eddington-critical limit and driven by "accretion wind evolution". Aims.We seek to derive the variations in the characteristic time scales of the long-term optical light curve and to determine the implications for the physical parameters of the system. Methods.We used existing and new optical monitoring observations covering a total time span of 14 years and compared the durations of the low and high states with the model calculations of Hachisu & Kato. Results.The cycle lengths and especially the durations of the optical high states show a longterm modulation with variations that, according to the accretion wind evolution model, would imply variations in the mass transfer rate by a factor of 5 on timescales of years.