  We have studied the future evolution of a recently discovered ULX source P13
in NGC7793. This source was shown to contain a 5-15$M_{\odot}$ black hole and a
massive 18-23$M_{\odot}$ B9Ia companion on a 64 day orbit. For low black hole
mass (5-10$M_{\odot}$) and high companion mass $\gtrsim 20 M_{\odot}$ the
binary is predicted to initiate a common envelope evolution in near future and
significantly decrease the orbital separation (6 hr orbit). This leads to a
high probability ($\sim70\%$) of the system surviving a supernova explosion
that will form a neutron star out of the companion. About one third of the
surviving BH-NS systems will merge within Hubble time and be a source of high
frequency gravitational radiation. We estimate that the chances of detection of
BH-NS systems with advanced LIGO/Virgo that form via P13-evolutionary channel
are at the level of 0.1 yr$^{-1}$ with wide range of allowed probability $0
\div 0.6$yr$^{-1}$. This is the fourth empirical estimate of BH-NS merger rate.
