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We report new CHARA/MIRC interferometric observations of the Cepheidarchetype $\delta$ Cep, which aimed at detecting the newly discoveredspectroscopic companion. We reached a maximum dynamic range $\Delta H $ = 6.4,5.8, and 5.2 mag, respectively within the relative distance to the Cepheid $r <25$ mas, $25 < r < 50$ mas and $50 < r < 100$ mas. Our observations did notshow strong evidence of a companion. We have a marginal detection at $3\sigma$with a flux ratio of 0.21\%, but nothing convincing as we found other possibleprobable locations. We ruled out the presence of companion with a spectral typeearlier than F0V, A1V and B9V, respectively for the previously cited ranges$r$. From our estimated sensitivity limits and the Cepheid light curve, wederived lower-limit magnitudes in the $H$ band for this possible companion tobe $H_\mathrm{comp} > 9.15, 8.31$ and 7.77 mag, respectively for $r < 25$ mas,$25 < r < 50$ mas and $50 < r < 100$ mas. We also found that to be consistentwith the predicted orbital period, the companion has to be located at aprojected separation $< 24$ mas with a spectral type later than a F0V star.

monthly notices of the royal astronomical society

Multiplicity of Galactic Cepheids from long-baseline interferometry~III.  Sub-percent limits on the relative brightness of a close companion of  $\delta$~Cephei