The duos merged and became The Mystic Four, with Cees and Piet Veerman on vocals and guitar, Schilder on guitar (and piano), and Muhren on bass.
By 1965 they changed their name to The Blue Cats, a reference to the colour of their suits and Cees' nickname, Poes (Dutch for 'cat').
In 2011, Kotak teamed up with Simple Plan to record a new version of their song, "Jet Lag". The Jakarta Post on , stated: "One of Indonesia’s most loved rock bands, Kotak, has just released its Best album, containing nine of the most popular tracks from the band’s previous albums, three new songs and a bonus track." The song “Jet Lag”, produced with Canadian pop-punk band Simple Plan, is also available in this album. Tantri, the vocalist, received the Best Rock Band/Group and Best Rock Album awards at AMI in 2009.
It was, however, a thinly-disguised solo-performance by Cees.
Late in 1979 members of The Cats decided to disband.
Dropping the 'Blue' from their name in 1966 they recruited drummer Theo Klouwer (30 June 1947 – 8 February 2001).
The Cats borrowed money from Jan Buys, who was later to become their manager, and recorded their first singles that immediately entered the charts.
Singing in English thanks to a songwriting duo from England, the group sounded British.