Abstract

Aims. 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. 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 eta Aql, within a field of view of 1,7 x 1.7 (3.4 x 3.4 for eta Aql), Results. We were able to reach contrast Delta N = 5-8 mag and Delta K, = 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 eta Aql at r > 0.2, we ruled out the presence of companions with a spectral type that is earlier than a B7V, A9V, A9V, AIN, 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 eta Aql at projected separation p 654.7 +/- 0.9 mas and a position angle PA = 92.8 +/- 0.1 degrees. We estimated its dereddened apparent magnitude to be m 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 eta Aql system.