A Cepheid is no more: Hubble's variable 19 in M33

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 t he 20th century, the variable, designated as V19, exhibited a 54.7 day peri od, an intensity-weighted mean B magnitude of 10.59 +/- 0.23 mag, and a B a mplitude of 1.1 mag. Its position in the period-luminosity plane was consis tent with the relation derived by Hubble from a total of 35 variables. Mode rn observations by the DIRECT project show a dramatic change in the propert ies 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 remain s a mystery. It is not clear how frequent such objects are or how often the y could be mistaken for classical Cepheids.