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 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 +/- 0.03 M-circle dot and R-p = 18.51(-0.07)(+0.12) R-circle dot; for the secondary, M-s = 7.3 +/- 0.3 M-circle dot and R-s = 4.02(-2.0)(+2.3) R-circle dot. Modelling the evolution of the system with MESA, we found an age of similar to 7.7 x 10(7) 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 aemission suggest the presence of a circumstellar or circumbinary disc. V1315 Cas will likely become a detached stripped star binary.

Automated summary

The search for dormant black holes has been ongoing for decades. Ellipsoidal variables are believed to be one of the best ways to find such black holes, but they are not a guaranteed method. In this study, the discovery of a new semidetached interacting binary, V1315 Cas, which was initially identified as an ellipsoidal variable, is reported. Through various analyses, characteristics such as the masses, radii, and evolutionary age of the binary system were determined. The study also suggests the presence of carbon depletion, nitrogen overabundance, and circumstellar or circumbinary disc in the system.