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, \(q=M_x/M_v\) is \(\gtrsim 0.8\). For an optical star mass \(M_v\sim 10 M_\odot\) the compact object in SS433 is a black hole with mass \(M_{BH}\gtrsim 8 M_\odot\). 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 \(dM_v/dt\sim 10^{-4}-3\times 10^{-5} M_\odot\) per year and an optical star mass \(M_v \sim 10-15 M_\odot\) 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.