PROPERTIES OF M31. III. CANDIDATE BEAT CEPHEIDS FROM PS1 PANDROMEDA DATA AND THEIR IMPLICATION ON METALLICITY GRADIENT. - Lee, C.-H.; Kodric, M.; Seitz, S.; Riffeser, A.; Koppenhoefer, J.; Bender, R.; Hopp, U.; Gössl, C.; Snigula, J.; Burgett, W. S.; Chambers, K. C.; Flewelling, H.; et al. (Astrophysical Journal, 2013)