The Extragalactic Wolf‐Rayet/Black‐Hole X‐ray Binary Candidates NGC 300 X‐1 and IC 10 X‐1

NGC 300 X‐1 and IC 10 X‐1 have recently been identified in nearby galaxies as extragalactic examples of the rare class of Wolf‐Rayet/compact object X‐ray binary system uniquely exemplified in the Galaxy by Cyg X‐3. Using Swift TOO observations of NGC 300 X‐1 and IC 10 X‐1, we recently discovered very similar periods of 32.8±0.4 and 34.8±0.9 hours, respectively. We believe that these are likely to be orbital periods. It seems surprising that these are so similar and large compared to the short 4.8 hour period of Cyg X‐3, suggesting different evolutionary paths. For both extragalactic candidates we show the allowed ranges of component masses and Wolf‐Rayet star wind velocities for which accretion disks could form around the black holes.