Enhancing the cosmic-ray mass sensitivity of air-shower arrays by combining radio and muon detectors Ewa M. Holt, Frank G. Schröder and Andreas Haungs Eur. Phys. J. C, 79 5 (2019) 371 Published online: 29 April 2019 DOI: 10.1140/epjc/s10052-019-6859-4