Locks on rollback segments only

Hello,
we are experiencing a strange behaviour on Oracle 7.3.3 where some sessions have locks on rollback segments only, no row locks on tables.

I have thought that if there is a lock on rollback segment owned by SID it means that this SID is writing to database and there should be locks on tables (or rows in table) owned by the very same SID.

Here is information of a sample lock extracted from OEM's Lock Manager: Lock Type: TX
Mode Held: Exclusive
Object Name: RS1
Object Type: ROLLBACK SEGMENT

Thanks in advance

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Timo Haatainen
TT-Tieto Oy