  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
$P_{\rm{orb}}$=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 \textsc{phoebe}2, we derived the following masses and radii:
for the primary, $M_p =0.84 \pm 0.03 \, M_\odot$ and $R_p
=18.51^{+0.12}_{-0.07} \, R_\odot$; for the secondary, $M_s =7.3 \pm 0.3
\,M_\odot$ and $R_s =4.02^{+2.3}_{-2.0}\,R_\odot$. Modeling the evolution of
the system with MESA, we found an age of $\sim$7.7e7 years. The system is at
the end of a period of rapid non-conservative mass transfer that reversed its
mass ratio, while significantly widening its orbit. The primary shows carbon
depletion and nitrogen overabundance, indicative of CNO processed material
being exposed due to mass transfer. An infrared excess as well as stationary
H$\alpha$ emission suggest the presence of a circumstellar or circumbinary
disc. V1315 Cas will likely become a detached stripped star binary.
