  X-ray binary systems are very popular objects for astrophysical
investigations. Compact objects in these systems are neutron stars, white
dwarfs and black holes. Neutron stars and white dwarfs can have intrinsic
magnetic fields. There is well known, famous theorem about absence of intrinsic
magnetic fields of black holes. But magnetic field can exist in the accretion
disk around a black hole. We present here the real estimates of the magnetic
field strength at the radius of innermost stable orbit in an accretion disk of
stellar mass black holes.
