Evolutionary Increase of the orbital Separation and Change of the Roche Lobe Size in SS433

We present results of long-term photometric monitoring of SS433 which proves a secular evolutionary increase of the orbital period of SS433 at a rate of $(1.14\pm 0.25)\times {10}^{-7}$ s~s${}^{-1}$. Using a physical model of non-conservative mass transfer in SS433 through a supercritical accretion disc around the compact companion, we reliably confirm that the binary mass ratio in SS433, $q=M_x\mathrm{/}{M}_v$ is $\gtrsim 0.8$. For an optical star mass $M_v\sim 10M_{\odot }$ the compact object in SS433 is a black hole with mass $M_{BH}\gtrsim 8M_{\odot }$. We discuss evolutionary implications of the found orbital period increase in SS433 – a secular change in the orbital separation and a size of the Roche lobe of the optical star. We show that for the mass-loss rate $d{M}_v\mathrm{/}dt\sim {10}^{-4}-3\times {10}^{-5}{M}_{\odot }$ per year and an optical star mass $M_v\sim 10-15M_{\odot }$ the found orbital period increase implies the corresponding orbital separation increase while the Roche lobe size can shrink or expand around a mean constant value depending on the optical star mass-loss rate which may be modulated with the precessional period.