Regularity of a double null coordinate system for Kerr-Newman-de Sitter spacetimes

We construct a double null coordinate system $(u,v,\theta_\star,\phi_\star)$ for Kerr-Newman-de Sitter spacetimes and prove that the two-spheres given by the intersection of the hypersurfaces $u=\mbox{constant}$ and $v=\mbox{constant}$ are $C^\infty$ in Boyer-Lindquist coordinates (including at the "poles"). The null coordinates allow one to immediately extend some results previously proven for Kerr. As an example, we illustrate how Sbierski's result, for the wave equation on the black hole interior, for Reissner-Nordstr\"{o}m and Kerr spacetimes, applies to Kerr-Newman-de Sitter spacetimes.
Comment: 55 pages, 7 figures, 29 references; v3: minor changes, matches final published version