Discovery of orbital Eccentricity and Evidence for orbital Period Increase of SS433

The examination of long-term (1979-2020) photometric observations of SS433 enabled us to discover a non-zero orbital eccentricity of $e=0.05\pm 0.01.$ We have also found evidence for a secular increase in the orbital period at a rate of $\dot P_\mathrm{b}=(1.0\pm0.3)\times10^{-7}$ s s$^{-1}$. The binary orbital period increase rate makes it possible to improve the estimate of the binary mass ratio $q=M_\mathrm{X}/M_\mathrm{V}>0.8$, where $M_\mathrm{X}$ and $M_\mathrm{V}$ are the masses of the relativistic object and the optical star, respectively. For an optical star mass of 10${\rm M}_\odot$, the mass of the relativistic object (a black hole) is $M_\mathrm{X}>8{\rm M}_\odot$. A neutron star in SS433 is reliably excluded because in that case the orbital period should decrease, in contradiction to observations. The derived value of $\dot P_\mathrm{b}$ sets a lower limit on the mass-loss rate in the Jeans mode from the binary system $\gtrsim 7\times 10^{-6} {\rm M}_\odot\mathrm{yr}^{-1}$. The discovered orbital ellipticity of SS433 is consistent with the model of the slaved accretion disc tracing the precession of the misaligned optical star's rotational axis.