I'm having trouble understanding the arguments in http://arxiv.org/abs/hep-th/9510209(adsbygoogle = window.adsbygoogle || []).push({});

They propose that M-theory on the orbifold [itex]R^9 \times S^1 \times S^1 / Z_2[/itex](1) yields Type I' on [itex]R^9 \times S^1[/itex], wheras M-theory on [itex]R^9 \times S^1 / Z_2 \times S^1[/itex](2) is [itex]E_8 \times E_8[/itex] on [itex]R^9 \times S^1[/itex]. What exactly is the difference between (1) and (2)? Does it imply a different ordering of applying the orbifold and the circular compactification? In other words: what happens when the two last factors are exchanged?

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# Question about SO(32) heterotic - Type I duality, M-theory perspective

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