We present the results of a high-resolution imaging survey for brown dwarf binaries in two open clusters .
The observations were carried out with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope .
Our sample consists of 8 brown dwarf candidates in \alpha Persei and 25 brown dwarf candidates in the Pleiades .
We have resolved 4 binaries in the Pleiades with separations in the range 0 \arcsec .094–0 \arcsec .058 , corresponding to projected separations between 11.7 AU and 7.2 AU .
No binaries were found among the \alpha Persei targets .
Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster , and for one of them we report the detection of H _ { \alpha } in emission and LiI absorption obtained from Keck II/ESI spectroscopy .
One of the binaries does not have a proper motion consistent with Pleiades membership .
We estimate that brown dwarf binaries wider than 12 AU are less frequent than 9 % in the \alpha Persei and Pleiades clusters .
This is consistent with an extension to substellar masses of a trend observed among stellar binaries : the maximum semimajor axis of binary systems decreases with decreasing primary mass .
We find a binary frequency of 2 binaries over 13 brown dwarfs with confirmed proper motion membership in the Pleiades , corresponding to a binary fraction of 15 ^ { +15 } _ { -5 } % .
These binaries are limited to the separation range 7-12 AU and their mass ratios are larger than 0.7 .
The observed properties of Pleiades brown dwarf binaries appear to be similar to their older counterparts in the solar neighborhood .
The relatively high binary frequency ( \geq 10 % ) , the bias to separations smaller than about 15 AU and the trend to high mass ratios ( q \geq 0.7 ) are fundamental properties of brown dwarfs .
Current theories of brown dwarf formation do not appear to provide a good description of all these properties .