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CONTEXT. Massive stars have high-multiplicity fractions, and many of themhave still undetected components, thus hampering the study of their properties.AIMS. I study a sample of massive stars with high angular resolution to bettercharacterize their multiplicity. METHODS. I observed 138 fields that include atleast one massive star with AstraLux, a lucky imaging camera at the 2.2 m CalarAlto telescope. I also used observations of 3 of those fields with ACS/HRC onHST to obtain complementary information and to calibrate the AstraLux data. Theresults were compared with existing information from the Washington Double StarCatalog, Tycho-2, 2MASS, and other literature results. RESULTS. I discover 16new optical companions of massive stars, the majority of which are likely to bephysically bound to their primaries. I also improve the accuracy for theseparation and magnitude difference of many previously known systems. In a fewcases the orbital motion is detected when comparing the new data with existingones and constraints on the orbits are provided. CONCLUSIONS. The analysisindicate that the majority of the AstraLux detections are bound pairs. For arange of separations of 0.1"-14" and magnitude differences lower than 8, I findthat the multiplicity fraction for massive stars is close to 50%. When objectsoutside those ranges are included, the multiplicity fraction should beconsiderably higher.

High-resolution imaging of Galactic massive stars with AstraLux. I. 138 fields with delta > -25 degrees