Supersoft X-ray sources (SSSs) are thought to be accreting white dwarfs (WDs) in close binary systems, with thermonuclear burning on their surfaces. The SSS RX J$0513.9-6951$ in the Large Magellanic Cloud (LMC) exhibits cyclic variations between optical low and high states, which are anti-correlated with its X-ray flux. This behaviour is believed to be the result of the periodic expansion and contraction of the WD due to variations in the accretion rate in the system. We analyse the eight high-resolution XMM and six grating Chandra spectra of RX J$0513.9-6951$ with our grid of model atmosphere spectra of hot WDs computed under the assumption of local thermodynamic equilibrium. Our aim is to test a contraction model of the source variability by tracking the evolution of the WD properties. We use a recently computed grid of hot WD model atmospheres, spanning a wide range of effective temperatures (Teff=100-1000 kK in steps of $25 kK$) and eight values of surface gravity. The LMC chemical composition of the atmospheres was assumed. The obtained fitting parameters (effective temperature Teff, surface gravity log and bolometric luminosity L) evolve on the Teff - and Teff - L planes. This evolution follows the model tracks of WDs with masses of 1.05-1.15,M_ and thermonuclear burning on the surface. We show that, when the source has a relatively small photospheric radius and is optically bright, it lies below the stable-burning strip with a relatively low bolometric luminosity. Conversely, the fainter optical states correspond to higher bolometric luminosity and larger photospheric radii of the hot WD. RXJ0513 lies within the stable-burning strip during this state. This means that the optical brightness of the system is lower when the WD is larger, more luminous, and illuminates the accretion disc more effectively. These results contradict the contraction model, which predicts the opposite behaviour of the source. We use a model that assumes that the far UV/soft X-ray flux is reprocessed into the optical band due to multiple scattering in the cloud system above the accretion disc. More significant illumination can lead to rarefying of the cloud slab, thereby reducing the reprocessing efficiency and making the source fainter in the optical band.

Examining the evolution of the supersoft X-ray source RX J0513.9-6951