  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
0.3" to 26.2", 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\pm7%. These results are
presented in the first of a series of papers based on our Volume-limited A-Star
(VAST) survey -- a comprehensive study of the multiplicity of A-type stars.
