, H. Liszt 
Institut de Radioastronomie Millimétrique,
300 Rue de la Piscine, F-38406 Saint Martin d'Hères,
France National Radio Astronomy Observatory,
520 Edgemont Road, Charlottesville, VA, USA 22903-2475 
Figure: H12,C16,O 
, H13,CO ( 
),
H12,C14,N, H13,C14,N ( 
), H14,N12,C, and
12,C32,S (J=2-1) absorption spectra toward B0415+379=3C111
(solid lines) with H12,C18,O ( 
),
H12,C15,N ( 
), H14,N13,C ( 
) and
C34,S overlaid (shaded).
We have measured galactic 
mm absorption spectra of HCO ,
HCN, HNC, and CS toward compact extragalactic continuum sources in
order to derive the isotopic abundance ratios 
C/ 
C, 
N/ 
N, 
O/ 
O,
and 
S/ 
S in local diffuse clouds. For carbon, our data confirm
recent results for the local ISM: we find C/ C = 
. For
nitrogen, we find 
consistent with the Solar
value of 270, but substantially smaller than the values inferred
from HCN emission in dense clouds. For sulfur we find S/ S\
consistent with the Solar value of 23. We also find
one striking individual anomaly: toward 3C111 (B0415+379),
H12,CN/H13,CN = 170 
in one kinematic component
(Fig. 3). We attribute this to the fractionation of
13,C into 13,CO which may be so great that HCN is starved
for 13,C.
Preprints:http://iram.fr/ lucas/papers/isotopes.ps.gz