  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.
