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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 arcsec, 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   per cent. The corresponding control sample multiple star frequency is three times lower at  12 +4  −3   per cent – a difference of 31 ± 7 per cent. 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.

The Volume‐limited A‐Star (VAST) survey – I. Companions and the unexpected X‐ray detection of B6–A7 stars