2014A&A...567A..60G

High-resolution imaging in several photometric bands can provide color and astrometric information of the wide-orbit component of Cepheid stars. Such measurements are needed to understand the age and evolution of pulsating stars. In addition, binary Cepheids have the potential to provide direct and model-independent distances and masses. We used the NAOS-CONICA adaptive optics instrument (NACO) in the near-infrared to perform a deep search for wide components around the classical Cepheids, Y Oph, FF Aql, X Sgr, W Sgr, and η Aql, within a field of view of 1.7''x1.7'' (3.4''x3.4'' for η Aql). We were able to reach contrast ΔH=5-8mag and ΔK_s=4-7mag in the radius range r>0.2'', which enabled us to constrain the presence of wide companions. For Y Oph, FF Aql, X Sgr, W Sgr, and η Aql at r>0.2'', we ruled out the presence of companions with a spectral type that is earlier than a B7V, A9V, A9V, A1V, and G5V star, respectively. For 0.1''< r < 0.2'', no companions earlier than O9V, B3V, B4V, B2V, and B2V star, respectively, are detected. A component is detected close to η Aql at projected separation ρ=654.7±0.9mas and a position angle PA=92.8±0.1°. We estimated its dereddened apparent magnitude to be m_H⁰=9.34±0.04 and derived a spectral type that ranges between an F1V and an F6V star. Additional photometric and astrometric measurements are necessary to better constrain this star and check its physical association to the η Aql system.