'Brady Bunch' Star -- My Cat Died ... Just Like the Professor

Yesterday was a bad day for sitcom stars ... TMZ has learned Cindy Brady's cat died -- just like The Professor from "Gilligan's Island" and Dave Madden from "The Partridge Family."

Susan Olsen's cat -- Sacha Baron Kitteh -- got seriously ill back in December ... and passed away Thursday after battling valiantly for a month.

Following the death ... Olsen asked an org she works with to set up a donations page for anyone looking to make a contribution in Sacha's honor.