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,[7] and is one of the most exciting and instructive events in variable-star astronomy.[8] The star has varied between apparent visual magnitude 12.3 and 14.3.[4]
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,[7] 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.[8] 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.[3]
In 1973, the spectral class of V725 Sagittarii was estimated to be between F8 and G2 and similar to a type Ib supergiant.[3] In 1994 it was observed to be G8 based on the spectral lines of metals and later than F8 based on the hydrogen lines.[9] In 2006, it was reported that in 2000 V725 Sagittarii was an early M star with emission lines.[10] In 2010, the spectral type was estimated from its colours and other properties to be K4III, although possibly late K.[3]
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