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Cabibbo-suppressed semileptonic decays provide an exciting laboratory
for studying charm decays. These decays provide information on the ratio
of CKM elements 
. For decays to the vector final states,
, measurement of the form-factor ratios can be related
directly to the same final states in 
decays which can allow extraction
of V
from 
decays.
Already, the decays 
and
have been observed in CLEO II[48] and are
providing important information on the form factors in charm decay.
The approach is to use the decay chain 
or
as a means to tag the 
or 
and to
reduce background. Presently, the 
data sample is the
largest in the world and a clean signal 
has been observed.
Other Cabibbo-suppressed decays are obscured by their Cabibbo-favored
counterparts. The 
signal is shown in
Fig. 
and is compared with the similar final state 
. From
these two signals CLEO finds 
=0.21
0.020.02, where the
second error results from the theoretical uncertainty in the value of the
form-factor ratio at zero momentum transfer. Extending the semileptonic decay
studies to the rarer 
and 
final states requires excellent
tagging efficiency and high luminosity, both of which will be
available to CLEO III.