1. C. P. Bachas, Class. Quant. Grav. 17, 951 (2000) [hep-th/0001093].
2. C. Bachas, in Fundamental Interactions: From Symmetries to Black Holes, J.-M. Frère et al eds., Brussels 1999, [hep-th/9907023].
3. C. P. Bachas, hep-ph/0003259.
4. I. Antoniadis, hep-th/9909212.
5. I. Antoniadis and A. Sagnotti, Class. Quant. Grav. 17, 939 (2000) [hep-th/9911205].