Static and spherically symmetric vacuum spacetimes with non-expanding principal null directions in f(R) gravityAlberto Guilabert, Pelayo V. Calzada, Pedro Bargueño and Salvador Miret-ArtésEur. Phys. J. C, 84 7 (2024) 678DOI: https://doi.org/10.1140/epjc/s10052-024-13063-3