from
Soviet Combat Aircraft of the Second World War Vol 1: Single Engined Fighters, by Yefim Gordon and Dmitri Khazanov, ISBN # 1-85780-083-4
FYI this is the single best source on VVS fighters during WWII, and its written by Russian Authors from Soviet Archives.
Yak-9UFirst flights in January 1944, with official test flights to follow in January-February 1944.
The 42nd Guard Fighter Air Regiment, commanded by Y Alexandrovich, was among the first units to receive the Yak-9U..... Wide spread use of the new fighters began at the end of the Summer of 1944 ..... During the tests, which continued almost to the end of 1944, 398 flights were made by the Yak-9U's and 299 air combats took place. According to the Regiments documentation the combat loss ratio was 28 to 2 in the Yak's favour..... A total of 3,921 Yak-9U's was built, about 2,500 of them before the end of the war, and the type was widely used on all fronts.
Yak-9UT (from the same source)
Testing of the Yak-9UT began in March 1945 and was completed, very successfully, after the wars end. But many of the 282 built took part in the air combats over Berlin during the last days of the war.
If you want the complete text, I suggest buying the book

Its well worth it