Null gravitational redshift by a Reissner–Nordström black hole in the strong field limit Guansheng He, Chaohong Pan, Xia Zhou, Weijun Li and Lin Li Eur. Phys. J. C, 80 6 (2020) 512 Published online: 08 June 2020 DOI: 10.1140/epjc/s10052-020-8085-5