Searching for visual companions of close Cepheids

Aims. High-resolution imaging in several photometric bands can provide color and astrometric information of the wide-orbit com-ponent 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. Methods. 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 (FoV) of 1.7′ ′ × 1.7′′ (3.4′ ′ × 3.4′ ′ for η Aql). Results. We were able to reach contrast ∆H = 5–8 mag and ∆Ks = 4–7 mag 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 ± 0.9 mas and a position angle PA = 92.8 ± 0.1◦. We estimated its dereddened apparent magnitude to be m0H = 9.34 ± 0.04 and derived a spectral type that ranges between an F1V and F6V star. Additional photometric and astrometri