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We present the results of medium-resolution spectroscopy of five nearbyHerbig Ae/Be stars at 4.7 micron: UX Ori, HD 34282, HD 50138, V380 Ori, HK Ori.The goal was to search for CO fundamental ro-vibrational emission. None of thetargets show CO features, either in absorption nor in emission. We derive a5sigma upper limit of < 10^-12 cm^-2 to the column density of hot CO (T ~ 1500K) in the sources. These upper limits are considerably lower than the values ofHerbig Ae/Be stars for which warm and hot CO emission has been reported.Thenon-detection of CO v=1-0 emission in these five targets suggest that HerbigAe/Be stars are not a homogeneous group with respect to the structure of thegaseous disk and/or the amount of CO in the inner 50 AU of their disks.

Upper limits on CO 4.7 μm emission from disks around five Herbig Ae/Be stars