Can the 5brane turn into a fundamental string?
There are M-theory compactifications where the 5-brane can wrap 4 compact directions to give an object that is effectively a string in the limit where the size of the compact directions is very small. S-duality on these compactifications will switch the membrane and 5-brane so could lead to the identification of a wrapped 5-brane with the fundamental string in some dual theory.
Ok, consider the following....
In M theory, the elementary string that creates all particles is formed when an M2 brane is wrapped tightly around the circular dimensions. Now, when the M2 turns into a string, can the string that is open (with end points) attach its end points to an M5 brane? I have read in only few sources that they can
Yes, M-theory on a circle is dual to the IIA string theory. The wrapped M2-brane is identified with the fundamental string and the wrapped M5-brane is identified with a D4-brane. So a string ending on a D4-brane is identified with an M2-brane ending on an M5-brane. So we learn that, even in 11d, an M2-brane can attach to an M5-brane. The reference http://arxiv.org/abs/hep-th/0412310 describes a solution for this system.
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