A Cepheid is no more: Hubble's variable 19 in M33
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We report on the remarkable evolution in the light curve of a variable star discovered by Hubble in M33 and classified by him as a Cepheid. Early in the 20th century, the variable, designated as V19, exhibited a 54.7 day period, an intensity-weighted mean B magnitude of 19.59 0.23 mag, and a B amplitude of 1.1 mag. Its position in the period-luminosity plane was consistent with the relation derived by Hubble from a total of 35 variables. Modern observations by the DIRECT project show a dramatic change in the properties of V19: its mean B magnitude has risen to 19.08 0.05 mag, and its B amplitude has decreased to less than 0.1 mag. V19 does not appear to be a classical (Population I) Cepheid variable at present, and its nature remains a mystery. It is not clear how frequent such objects are or how often they could be mistaken for classical Cepheids.