Context :

Aims : We report on archival Hubble Space Telescope ( HST ) observations of the globular cluster M71 ( NGC 6838 ) .

Methods : These observations , covering the core of the globular cluster , were performed by the Advanced Camera for Surveys ( ACS ) and the Wide Field Planetary Camera 2 ( WFPC2 ) .
Inside the half-mass radius ( r _ { \mathrm { h } } = 1 \aas@@fstack { \prime } 65 ) of M71 , we find 33 candidate optical counterparts to 25 out of 29 Chandra X-ray sources while outside the half-mass radius , 6 possible optical counterparts to 4 X-ray sources are found .

Results : Based on the X-ray and optical properties of the identifications , we find 1 certain and 7 candidate cataclysmic variables ( CVs ) .
We also classify 2 and 12 X-ray sources as certain and potential chromospherically active binaries ( ABs ) , respectively .
The only star in the error circle of the known millisecond pulsar ( MSP ) is inconsistent with being the optical counterpart .

Conclusions : The number of X-ray faint sources with L _ { \mathrm { X } } > 4 \times 10 ^ { 30 } ergs s ^ { -1 } ( 0.5–6.0 keV ) found in M71 is higher than extrapolations from other clusters on the basis of either collision frequency or mass .
Since the core density of M71 is relatively low , we suggest that those CVs and ABs are primordial in origin .