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The C-star envelope IRC+10216 harbors a rich variety of molecules, with morethan 80 detected to date. During the course of a $\lambda$ 3 mm survey ofIRC+10216 carried out with the IRAM 30-m telescope we have detected variousweak lines, with antenna temperatures of a few mK, that we assign to rotationaltransitions of four new molecules. The observation of three lines of C$_5$Sconfirms a previous tentative identification of this molecule by Bell et al.(1993) based on a line at 24.0 GHz. We also report the tentative identificationof three molecules not yet observed in space: MgCCH, the first metal acetylidedetected in space, and NCCP and SiH$_3$CN, the phosphorus and silicon analogsof cyanogen (NCCN) and methyl cyanide (CH$_3$CN). We derive the followingcolumn densities: $N$(C$_5$S) = (2-14) $\times$ 10$^{12}$ cm$^{-2}$ (dependingon the rotational temperature adopted), $N$(MgCCH) = 2 $\times$ 10$^{12}$cm$^{-2}$, $N$(NCCP) = 7 $\times$ 10$^{11}$ cm$^{-2}$, and $N$(SiH$_3$CN) =10$^{12}$ cm$^{-2}$. The S-bearing carbon chain C$_5$S is less abundant thanC$_3$S, while MgCCH has an abundance in between that of MgNC and those of MgCNand HMgNC. On the other hand, NCCP and SiH$_3$CN are the least abundant P- andSi-bearing molecules observed to date in IRC+10216. Based on the behavior ofsimilar molecules it is likely that these four species are formed in the outercircumstellar layers of IRC+10216. We discuss possible gas-phase formationroutes.

New molecules in IRC+10216: confirmation of C$_5$S and tentative identification of MgCCH, NCCP, and SiH$_3$CN