Top Undo Users

I am interested to know who had been top UNDO users - say in the past 24 hours(who may have already logged out). We do not seem to have any ASH views to get this info. Do we?

Currently I am using log off trigger to capture 'undo change vector size' from v$sesstat.

Thanks for your input, in advance.
Shiva

PS: I'm on 10GR1.

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