1 Introduction

Interstellar gas and dust form the basic ingredients from which planetary systems are built (e.g., van Dishoeck & Blake 1998; Ehrenfreund & Charnley 2000). In particular, the icy grains can agglomerate in the cold midplane of circumstellar disks to form planetesimals such as comets. In the cold (T< 20 K) and dense ( $n_{\rm H} = 10^6$-10⁹ cm^-3) regions of disks, all chemical models predict a strong freeze-out of molecules onto grain surfaces (e.g., Aikawa et al. 2002). The low molecular abundances in disks compared to those in dense clouds as derived from (sub)millimeter lines is widely considered to be indirect evidence for freeze-out (Dutrey et al. 1997; Thi et al. 2001).

Observations of gaseous and solid CO have been performed for a few transitional objects from class I to class II that are known to posses a disk. Boogert et al. (2002a) observed L 1489 in Taurus - a large 2000 AU rotating disk -, but the amount of solid CO is not exceptionally high ($\sim$7% of gaseous CO). This may stem from the fact that these systems are still far from edge-on (inclination $\sim$ $20{\hbox{$^\circ$ }}{-}30{\hbox{$^\circ$ }}$) so that the line of sight does not intersect the midplane, the largest reservoir of solid CO. Shuping et al. (2001) found strong CO depletion toward Elias 18 in Taurus, but both the disk stucture and its viewing angle are not well constrained. More promising targets are pre-main-sequence stars for which near-infrared images have revealed nebulosities separated by a dark lane (e.g. Padgett et al. 1999). The lane is interpreted as the cold midplane of a disk seen close to edge-on where visible and even near-infrared light are extremely extinct. Among such dark-lane objects, CRBR 2422.8-3423 is a red (H-K=4.7) low luminosity ( $L=0.36~L_{\odot}$, Bontemps et al. 2001) object surrounded by a near edge-on disk, discovered in images with the ESO Very Large Telescope (VLT) at 2 $\mu$m (Brandner et al. 2000). Its spectral energy distribution (SED) is consistent with that of a class I object or an edge-on class II object with strong silicate absorption at 9.6 $\mu$m. It is located at the edge of the $\rho$ Oph cloud complex in the core F, $\sim$30 $^{\prime\prime}$ west of the infrared source IRS 43 and a few arcmin south-east of Elias 29 (Motte et al. 1998).

This letter reports the detection of a large quantity of solid CO and the presence of gaseous CO in the line of sight of CRBR 2422.8-3423 using the ESO-VLT (Sects. 2 and 3). Possible contamination by foreground cloud material is considered in Sect. 4, followed by a discussion on the location and origin of the CO gas and dust in the disk (Sect. 5).