Refer cumulative or an incremental backup (export )
we have a parameter (RECORD) & the default parameter is yes & is to track the incremental backup of a table 
so those backup (export) information will get update in the following sys tables
SYS.INCEXP, SYS.INCFIL,SYS.INCVID

desc SYS.INCVID
 EXPID

desc SYS.INCEXP
 OWNER#
 NAME
 TYPE#
 CTIME
 ITIME
 EXPID


desc SYS.INCFIL

EXPID
EXPTYPE
EXPFILE
EXPDATE
EXPUSER


i have not checked any thing after an export from these tables try it out and let others also know about this


Regards

Rajesh RD