Bobs61,
I can confirm this - an F/O may call stop on any take-off (including one the Captain is handling) for:
1. any fire
2. an engine failure
3. the configuration warning
in addition, when handling the T/O, he may also call stop for:
4. a blocked runway
5. any serious handling difficulties
Why on earth does this shock you so much ?
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"After V1, we'll take the emergency into the air - You call 'Positive Climb' and I'll ask for the gear up. We'll climb away at between V2 and V2 + 25"