Denny Doherty was one of the founding members of The Mamas & Papas. Being a huge influence on harmonies of the time, The Cowsills were doubtlessly influenced by the beautiful sounds of The Mamas & Papas. And we can't forget that The Cowsills recorded "Monday, Monday" on their In Concert LP.
John Cowsill was also the drummer for a version of the M&P's in the 80's. There is audio of John playing with Denny, John Phillips, MacKenzie Phillips, and Spanky McFarlane.
Denny had an abdomenal aneurysm.