Previous studies have demonstrated that putatively single nitrogen-type Wolf-Rayet stars ( WN stars ) without known companions are X-ray sources .
However , almost all WN star X-ray detections so far have been of earlier WN2 - WN6 spectral subtypes .
Later WN7 - WN9 subtypes ( also known as WNL stars ) have proved more difficult to detect , an important exception being WR 79a ( WN9ha ) .
We present here new X-ray detections of the WNL stars WR 16 ( WN8h ) and WR 78 ( WN7h ) .
These new results , when combined with previous detections , demonstrate that X-ray emission is present in WN stars across the full range of spectral types , including later WNL stars .
The two WN8 stars observed to date ( WR 16 and WR 40 ) show unusually low X-ray luminosities ( L _ { x } ) compared to other WN stars , and it is noteworthy that they also have the lowest terminal wind speeds ( v _ { \infty } ) .
Existing X-ray detections of about a dozen WN stars reveal a trend of increasing L _ { x } with wind luminosity L _ { wind } = \frac { 1 } { 2 } \dot { M } v _ { \infty } ^ { 2 } , suggesting that wind kinetic energy may play a key role in establishing X-ray luminosity levels in WN stars .