Under "Offshore and Coastal Racing Rules 1993" Cats 1,2,3 (slightly
abridged)

"One Storm Jib of not larger than 5% of the square of the luff of the
largest headsail in area, the luff of which does not exceed 65% of the luff
of the largest headsail and of suitable strength for the purpose. Aromatic
polyamides, carbon fibres and other high modulus fibres shall not be used."

Jofra