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Published online by Cambridge University Press: 26 May 2016
Compared to several other Be/X-ray binaries, 4U1907+09 has been observed more frequently due to the fact that it is found in an ‘on’ state more often. It also has a short orbital period of ~ 8 days as compared to the long orbital periods commonly found in these binaries. But despite the attention it has received, the exact nature of the primary remains elusive. While some observers maintain it to be a Be/X-ray binary, others prefer to put it into the class of OB supergiants.