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, ho we ver, is not a panacea as we show here with one example. We indeed report the discovery of a new semidetached interacting binary, V1315 Cas, disco v ered as an ellipsoidal variable. Using data from photometric surv e ys (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 PHOEBE 2, we derived the following masses and radii: for the primary, M_p = 0.84 ± 0.03 M_☉  and R_p = 18. 51 + ^0.12 _−0.07 R_☉ ; for the secondary, M_s = 7. 3 ± 0.3 M_☉ and Rs = 4.02 ^+2.3−2.0 R_☉. 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-conserv ati ve mass transfer that reversed its mass ratio, while significantly widening its orbit. The primary shows carbon depletion and nitrogen o v erabndance, indicativ e 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.