Isocurvature Fluctuations of the M-theory Axion in a Hybrid Inflation   Model
  The M-theory, the strong-coupling heterotic string theory, presents various interesting new phenomenologies. The M-theory bulk axion is one of these. The decay constant in this context is estimated as $F_a\simeq 10^{16}$ GeV. Direct searches for the M-theory axion seem impossible because of the large decay constant. However, we point out that large isocurvature fluctuations of the M-theory axion are obtained in a hybrid inflation model, which will most likely be detectable in future satellite experiments on anisotropies of cosmic microwave background radiation. 
Toshiyuki Kanazawa, M. Kawasaki, Naoshi Sugiyama and T. Yanagida