The Radial Velocity Curve for He <scp>II</scp> Emission Cannot Be Used for Component Mass Determination in SS 433

Abstract More than 150 measurements of the He ii 4686 Å emission line in spectra of SS 433 were obtained during 388 nights in 2020–2025 with the Transient Double-beam Spectrograph on the 2.5 m telescope of Caucasian Mountain Observatory of Sternberg Astronomical Institute. We found that the He ii emission line formation region is not eclipsed and is significantly larger than both the donor star and the photosphere of the supercritical accretion disk. The He ii radial velocity curve was found to be independent of the precessional phase and inconsistent with the photometric curve. These findings suggest that the He ii line does not reflect the orbital motion of the compact object. Therefore, spectroscopic estimates of the masses of the components in SS 433 based on the He ii emission line can be unrealistic.