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We present the analysis of a sub-DLA system (log N(H^0)=20.0+/-0.15,z_abs=2.69) toward SDSS J123714+064759 (z_em=2.78). Using the VLT/UVES andX-shooter spectrographs, we detect H2, HD and CO molecules in absorption withlog N(H2,HD,CO)=(19.21,14.48,14.17). The overall metallicity of the system issuper-solar ([Zn/H]=+0.34) and iron is highly depleted ([Fe/Zn]=-1.39),revealing metal-rich and dusty gas. The strongest H2 component does notcoincide with the centre of the HI absorption. This implies that the molecularfraction in this component, f=2N(H2)/(2N(H2)+N(H^0)), is larger than the meanmolecular fraction <f>=1/4 in the system. This is supported by the detection ofCl^0 associated with this H2-component having N(Cl^0)/N(Cl^+)>0.4. Since Cl^0is tied up to H2 by charge exchange reactions, this means that the molecularfraction in this component is not far from unity. The size of the molecularcloud is probably smaller than 1pc. Both the CO/H2=10^-5 and CO/C^0~1 ratiosfor f>0.24 indicate that the cloud classifies as translucent, i.e., a regimewhere carbon is found both in atomic and molecular form. The correspondingextinction, Av=0.14, albeit lower than the definition of a translucentsightline (based on extinction properties), is high for the observed H^0 columndensity. This means that intervening clouds with similar local properties butwith larger column densities could be missed by current magnitude-limited QSOsurveys. The excitation of CO is dominated by radiative interaction with theCosmic Microwave Background Radiation (CMBR) and we derive Tex(CO)=10.5+0.8-0.6K when TCMBR(z=2.69)=10.05 K is expected. The astration factor of deuterium-with respect to the primordial D/H ratio- is only about 3. This can be theconsequence of accretion of unprocessed gas from the intergalactic medium ontothe associated galaxy. [abridged]

A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5