The paper under reviewing is one of the best in the actual gravitational research which is showing in its fullness the possibilities, complexities and beauty of the gravitation theory when combined with Maxwell and gauge fields. The authors begin chronologically and refer to a work which became actually classic: the well-known solution of A. I. Janis et al. [“Reality of the Schwarzschild singularity”, Phys. Rev. Lett. 20, No. 16, 878–880 (1968; doi:10.1103/PhysRevLett.20.878)] which provides us a solution coinciding with a Schwarzschild field out of the events horizon, and a pointlike source inside the events horizon. The JNW solution, which is reviewed in Chapter 4.1 of the article under examination, is not investigated as completely as other known black holes solutions, like Kerr, Reissner-Nordstrom or Kerr-Newman, but the authors base their article on very recent research, just published during the last few years, which show us that the interest to the Janis-Newman-Wincour solution is very great, so that we can expect an increasing number of research in this area in the next years.
Besides the interest to this research as to an exact mathematical physics research in the area of gravitation theory, unified quantum field theory it can open some new perspectives for astrophysical research due to its connection with black holes. Particularly, one of very intriguing problem can be the possible violation of the Cosmic censorship principle formulated by Penrose in naked singularities and its discussed actual relation with the so-called problem of “dark matter”.
The paper seems to be very rigorous, all the statements proved. Meanwhile, I have a suggestion of historical-biobliographical nature. Namely, in 1985–1986 the Romanian researcher, G. Maftei, deduced a new solution of Einstein-Maxwell equations entitled the [“Double Kerr-Schild solution”, Romanian J Phys. 30, No. 8, 647–657 (1985)]. One year later the same researcher investigated the particles (electrically and magnetically charged) orbits in this new metric, see [Romanian J. Phys. 31, No. 2, 107–120 (1986)]. It seems to me that a research of the likelihood of Maftei’s new solution and the reviewed article “double copy” new solution would be very useful. Perhaps, some limiting cases of two solutions coincide, at least. I should mention that Maftei investigated his new solution only classically, or in the framework of classical (non-quantum) physics.