The state fire marshall is investigating a house fire in Derby that killed a 70-year-old man and his pet.  A passerby walking their dog late evening on Christmas saw fire coming from the back of a duplex in the 700 block of E. Meadowlark called 911 and warned the neighbors.  Derby firefighters arrived to find fire coming out of the window of the unit, and after entering found the man in a bedroom along with his pet.  It is unclear if he lived there alone or with someone else.  The fire was contained to one room and the hallway and there was considerable smoke damage throughout the home.