When the disc's away, the stars will play: dynamical masses in the nova-like variable KR Aur with a pinch of accretion

Abstract

We obtained time-resolved optical photometry and spectroscopy of the nova-like variable KR Aurigae in the low state. The spectrum reveals a DAB white dwarf (WD) and a mid-M dwarf companion. Using the companion star's i-band ellipsoidal modulation we refine the binary orbital period to be P = 3.906519 ± 0.000001 h. The light curve and the spectra show flaring activity due to episodic accretion. One of these events produced brightness oscillations at a period of 27.4 min, that we suggest to be related with the rotation period of a possibly magnetic WD at either 27.4 or 54.8 min. Spectral modelling provided a spectral type of M4-5 for the companion star and $T_{1}=27\, 148$ $\pm \, 496$ K, $\log \, g=8.90 \pm 0.07$ , and $\log (\mathrm{He/H})= -0.79^{+0.07}_{-0.08}$ for the WD. By simultaneously fitting absorption- and emission-line radial velocity curves and the ellipsoidal light curve, we determined the stellar masses to be $M_1 = 0.94^{+0.15}_{-0.11}\, {\rm{M}_{\rm \odot}}$ and $M_2 = 0.37^{+0.07}_{-0.07}\,{\rm{M}_{\rm \odot}}$ for the WD and the M-dwarf companion, respectively, and an orbital inclination of $47^{+1^{\rm o}}_{-2^{\rm o}}$ . Finally, we analyse time-resolved spectroscopy acquired when the system was at an i-band magnitude of 17.1, about 1.3 mag brighter than it was in the low state. In this intermediate state, the line profiles contain an emission S-wave delayed by ≃0.2 orbital cycle relative to the motion of the WD, similar to what is observed in SW Sextantis stars in the high state.