Multiplicity of rapidly oscillating Ap stars

Context. Rapidly oscillating Ap (roAp) stars have rarely been found in binary or higher order multiple systems. This might have implications for their origin.
Aims: We intend to study the multiplicity of this type of chemically peculiar stars, looking for visual companions in the range of angular separation between 0.05 arcsec and 8''.
Methods: We carried out a survey of 28 roAp stars using diffraction-limited near-infrared imaging with NAOS-CONICA at the VLT. Additionally, we observed three non-oscillating magnetic Ap stars.
Results: We detected a total of six companion candidates with low chance projection probabilities. Four of these are new detections, the other two are confirmations. An additional 39 companion candidates are very likely chance projections. We also found one binary system among the non-oscillating magnetic Ap stars. The detected companion candidates have apparent K magnitudes between 6.8m and 19.5m and angular separations ranging from 0.23 arcsec to 8.9 arcsec, corresponding to linear projected separations of 30-2400 AU.
Conclusions: While our study confirms that roAp stars are indeed not very often members of binary or multiple systems, we have found four new companion candidates that are likely physical companions. A confirmation of their status will help understanding the origin of the roAp stars.

Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programme Nos. 079.D-0537(A) and 076.C-0170).Appendix A is available in electronic form at http://www.aanda.org