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±0.25)×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=MX/MV is ≳0.8. For an optical star mass MV∼10M⊙ the compact object in SS433 is a black hole with mass MBH≳8M⊙. 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 ṀV∼10−4−3×10−5M⊙ per year and an optical star mass MV∼10−15M⊙ 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. •Orbital period of SS433 increases at a rate of about 10−7 s/s.•Measured orbital period increase implies the binary mass ratio in SS433 q=Mx/Mv>0.8.•The compact object in SS433 is a black hole with Mx>8 solar masses.•For characteristic optical star mass-loss rate the orbital separation in SS433 increases.•Roche lobe of the optical star can be almost constant due to mass-loss variations.