Caught in the post-mass transfer phase: discovery that HD 46198 is a rare extreme mass ratio SB2 binary

ABSTRACT Spectroscopic monitoring by our TIGRE telescope with its intermediate-resolution spectrograph revealed the SB2 behaviour of the stellar system HD 46198. An analysis of the radial velocity measurements of the sharp-lined, low-mass (secondary) component yields an orbital period of $P=129.8$ d for the system, with a close to circular orbit. We then applied a modified version of our qer20 package spectral disentangling method to measure the small Doppler shifts of the much more massive and broader-lined (primary) component and obtained the orbital solution of the SB2 system, as well as the disentangled profile of the H $\alpha$ line for both stellar components. In particular, we find an extremely low mass ratio of $q=M_2/M_1=0.083$. We revised the Transiting Exoplanet Survey Satellite photometry data but did not find any indication of eclipses. A determination of the stellar parameters shows that the secondary component is already evolved ($\log (g)\approx 1.3$), so that the system is definitely not primordial. The system suffers currently no mass-overflow, because the radii are smaller than the distances to the Lagrange point L$_1$, and there is no X-ray emission. However, there must have been a significant exchange and loss of mass and angular momentum in a past, interactive phase of this binary, in which the orbit circularized. Hence, we conclude that HD 46198 is another extreme mass ratio post-mass transfer binary system.