V725 Sagittarii: From Population II Cepheid to Red Semiregular Variable

V725 Sgr is a unique pulsating variable star, first recognized by H. Swope, which increased its period from 12 to 21 days between 1926 and 1935. It was then neglected for three decades. We have used visual and photoelectric photometry of V725 Sgr from 1985 to 2004 to study its recent variability, using light‐curve, Fourier, least‐squares, and self‐correlation analyses. Whereas the period was 12 days in 1926, it is now about 90 days. The range is typically a magnitude, inV. Its spectrum and variability, which earlier were those of a Cepheid, are now those of a red semiregular variable. The star’s behavior is consistent with a thermal flash and blue loop in the H‐R diagram, from the asymptotic giant branch to the Cepheid instability strip and back again.