Observational Data

At present, the following facts are known about double degenerate compact binaries in our Galaxy:

    
Table: BH Candidates (Cherepashchuk [28]).
Table: Binary PSR with NS secondaries (Nice et al. [27])

1. A few binary radiopulsars are known to have the secondary NS component (Table ).
2. Three of these binary pulsars must coalesce due to the orbital angular momentum removal by GW on a time scale shorter than the age of the Universe (the Hubble time  yr).
3. No binary pulsars with BH is known as yet (although from evolutionary considerations one may expect one such an object to be formed in the Galaxy per about 1000 single pulsars, Lipunov et al. [14])
4. No binary BH has been found so far.
5. In contrast, 10 BH candidates are already known in X-ray binary systems with normal companions [28]. Their parameters are listed in Table . Note that the mean BH mass in these systems is < M , i.e. BH formed in stellar evolution are notably more massive than NS (with the typical mass 1.4M ).