Kate O'Mara Dead -- 'Dynasty' Star Dies at 74

Kate O'Mara -- who became a household name for playing Joan Collins' sister on "Dynasty" -- died Sunday following a short illness.

Kate's agent says the actress passed away in a Sussex, England nursing home this morning ... but it's unclear what illness she was suffering from.

O'Mara's manipulative character on "Dynasty" named Cassandra "Caress" Morell made her show debut in '86 ... but only appeared on the soap for 2 seasons.

More recently, Kate acted in a stage play in 2012.

She was 74.