The eastern Cepheus cloud, a nearby translucent and diffuse molecular cloud
of 
, has been fully mapped in
. A selection of positions has also been sampled at
the same resolution of 0.8 pc in 
, 
, and 
to cover the
full brightness range of the cloud. The radiative transfer of the lines,
under LTE or LVG assumptions, imply low excitation temperatures of 
in CO and 
in 
. The optical depth in CO is
moderate (
and 
) for CO
column-densities up to 
and the
cloud is optically thin to emission up to 
. For both isotopes, these conditions correspond
to fairly constant 
ratios, as observed over the cloud.
The 
) ratios have been found to
decrease with 
) because of the rapid saturation of the CO
lines. Significant fluctuations of the ratios about this relation have also
been observed which are comparable to the variations in ) or in 
reported in two other dark clouds, HCL 2 and IC 5146 on similar linear
scales (0.2 pc). Together they indicate that large fluctuations in abundance or excitation, by a factor up to 4, occur in the cloud
envelopes. The distribution of line widths or CO excitation temperatures
measured in Cepheus cannot account for the scatter in the ) ratios. The excitation conditions of the molecules are also quite stable and beam dilution of dense clumps
is not supported by the 2.3' data. So, the large fluctuations in intensity more likely reflect abundance variations along the line
of sight. Isotopic fractionation in the cold environment of Cepheus may
easily enhance the density locally. Detailed observations of
the fluctuations and of 
lines would provide
important clues to the photodissociation and fractionation processes in the
molecular cloud envelopes.
Acknowledgements
G.P.R. and I.A.G. wish to thank Tom Dame and Sam Palmer for their support and assistance during the observations, Alain Castets for lending us his LVG program, and the referee for his helpful remarks.