  Using all the RXTE archival data of Sco X-1 and GX 5-1, which amount to about
1.6 mega seconds in total, we searched for possible occultation events caused
by Oort Cloud Objects. The detection efficiency of our searching approach was
studied with simulation. Our search is sensitive to object size of about 300 m
in the inner Oort Cloud, taking 4000 AU as a representative distance, and of
900 m in the outer Oort Cloud, taking 36000 AU as the representative distance.
No occultation events were found in the 1.6 Ms data. We derived upper limits to
the number of Oort Cloud Objects, which are about three orders of magnitude
higher than the highest theoretical estimates in the literature for the inner
Oort Cloud, and about six orders higher for the outer Oort Cloud. Although
these upper limits are not constraining enough, they are the first obtained
observationally, without making any model assumptions about comet injection.
They also provide guidance to such serendipitous occultation event search in
the future.
