With an adaptive optics imaging survey of 148 B6-A7 stars , we have tested the hypothesis that unresolved lower-mass companions are the source of the unexpected X-ray detections of stars in this spectral type range .
The sample is composed of 63 stars detected in X-rays within the ROSAT All-Sky Survey and 85 stars that form a control sample ; both subsets have the same restricted distribution of spectral type , age , X-ray sensitivity and separation coverage .
A total of 68 companion candidates are resolved with separations ranging from 0 \aas@@fstack { \prime \prime } 3 to 26 \aas@@fstack { \prime \prime } 2 , with 23 new detections .
The multiple star frequency of the X-ray sample based on companions resolved within the ROSAT error ellipse is found to be 43 ^ { +6 } _ { -6 } \% .
The corresponding control sample multiple star frequency is three times lower at 12 ^ { +4 } _ { -3 } \% – a difference of 31 \pm 7 \% .
These results are presented in the first of a series of papers based on our V olume-limited A - St ar ( VAST ) survey – a comprehensive study of the multiplicity of A-type stars .