Everything that glitters is not gold: V1315 Cas is not a dormant black hole

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 semi-detached 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 Porb=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 $R_s =4.02^{+2.3}_{-2.0}\, \rm {R}_{\odot }$. Modeling the evolution of the system with MESA, we found an age of ∼7.7 × 107 years. 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 as well as stationary Hα emission suggest the presence of a circumstellar or circumbinary disc. V1315 Cas will likely become a detached stripped star binary.