2023MNRAS.524.5749Z

The quest for quiet or dormant black holes has been ongoing since several decades. Ellipsoidal variables possibly indicate the existence of a very high-mass invisible companion and are thought to be one of the best ways to find such dormant black holes. This, however, is not a panacea as we show here with one example. We indeed report the discovery of a new semidetached interacting binary, V1315 Cas, discovered as an ellipsoidal variable. Using data from photometric surveys (ASAS-SN, TESS) and high-resolution spectroscopy, we derived a nearly circular orbit with an orbital period of P_orb = 34.54 d. The binary system consists of an evolved F-type star primary that is likely still filling its Roche lobe and a B-type star secondary. Using PHOEBE2, we derived the following masses and radii: for the primary, $M_p =0.84 \pm 0.03 \, \rm {M}_{\odot}$ and $R_p =18.51^{+0.12}_{-0.07} \, \rm {R}_{\odot}$; for the secondary, $M_s =7.3 \pm 0.3 \, \rm {M}_{\odot}$ and $R_s =4.02^{+2.3}_{-2.0}\, \rm {R}_{\odot}$. Modelling the evolution of the system with MESA, we found an age of ∼7.7 × 10⁷ yr. The system is at the end of a period of rapid non-conservative mass transfer that reversed its mass ratio, while significantly widening its orbit. The primary shows carbon depletion and nitrogen overabundance, indicative of CNO-processed material being exposed due to mass transfer. An infrared excess and stationary H α emission suggest the presence of a circumstellar or circumbinary disc. V1315 Cas will likely become a detached stripped star binary.