Context : The C-star envelope IRC +10216 harbors a rich variety of molecules , with more than 80 detected to date .
During the course of a \lambda 3 mm survey of IRC +10216 carried out with the IRAM 30-m telescope we have detected various weak lines , with antenna temperatures of a few mK , that we assign to rotational transitions of four new molecules .
The observation of three lines of C _ { 5 } S confirms 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 identification of three molecules not yet observed in space : MgCCH , the first metal acetylide detected in space , and NCCP and SiH _ { 3 } CN , the phosphorus and silicon analogs of cyanogen ( NCCN ) and methyl cyanide ( CH _ { 3 } CN ) .
We derive the following column densities : N ( C _ { 5 } S ) = ( 2-14 ) \times 10 ^ { 12 } cm ^ { -2 } ( depending on 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 than C _ { 3 } S , while MgCCH has an abundance in between that of MgNC and those of MgCN and HMgNC .
On the other hand , NCCP and SiH _ { 3 } CN are the least abundant P- and Si-bearing molecules observed to date in IRC +10216 .
Based on the behavior of similar molecules it is likely that these four species are formed in the outer circumstellar layers of IRC +10216 .
We discuss possible gas-phase formation routes .

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