V725 Sagittarii is a variable star in the southern constellation of Sagittarius. As recently as a century ago, it was a Population II Cepheid; its transformation was documented by Henrietta Swope beginning in 1937, and is one of the most exciting and instructive events in variable-star astronomy. The star has varied between apparent visual magnitude 12.3 and 14.3.
Prior to 1926, this star showed the appearance of being an irregular variable. It then became a Population II Cepheid showing a regular light curve with a period of 12 days. Monitoring showed a gradual increase to a 21 day period by 1935, but did not show a corresponding change in brightness. The star was mostly ignored until 1967–68 when it was seen to vary by 0.4 magnitude with a 50 day period. Steady observation thereafter showed that the star had experienced a thermal flash and performed a loop on the H-R diagram. It migrated from the asymptotic giant branch (AGB) to the Cepheid instability strip and then back to the AGB.
In 1973, the spectral class of V725 Sagittarii was estimated to be between F8 and G2 and similar to a type Ib supergiant. In 1994 it was observed to be G8 based on the spectral lines of metals and later than F8 based on the hydrogen lines. In 2006, it was reported that in 2000 V725 Sagittarii was an early M star with emission lines. In 2010, the spectral type was estimated from its colours and other properties to be K4III, although possibly late K.