  Ultralight dark photons predicted in several Standard Model extensions can
trigger the superradiant instability around rotating black holes if their
Compton wavelength is comparable to the Blackhole radius. Consequently, the
angular momentum of the black hole is reduced to a value which depends upon the
mass and spin of the black hole as well as the mass of the dark photon. We use
the mass and spin measurements of the primary black holes in two recently
observed binary black hole systems: GW190517 and GW190426_152155 to constrain
dark photon mass in the ranges $1.7\times 10^{-14}{\rm\ eV}<m_{A'}<7.6\times
10^{-13}{\rm\ eV}$ and $1.3\times 10^{-13}{\rm\ eV}<m_{A'}<4.2\times
10^{-12}{\rm\ eV}$ respectively, assuming a timescale of a few million years
from the time of formation of the binary black hole system to the time of their
merger. We also discuss an interesting X-ray binary system, MAXI J1820_070,
albeit with a relatively small value of the spin parameter.
