A puzzling 2-hour X-ray periodicity in the 1.5-hour orbital period black widow PSR J1311−3430 Andrea De Luca, Martino Marelli, Sandro Mereghetti, Ruben Salvaterra, Roberto Mignani and Andrea Belfiore A&A, 667 (2022) L7 Published online: 11 November 2022 DOI: 10.1051/0004-6361/202244643