We have obtained a Hubble Space Telescope STIS ultraviolet high-dispersion echelle-mode spectrum of the binary companion of the double-mode classical Cepheid Y Car. The velocity measured for the hot companion from this spectrum is very different from reasonable predictions for binary motion, implying that the companion is itself a short-period binary. The measured velocity changed by 7 km s -1 during the 4 days between two segments of the observation, confirming this interpretation. We summarize "binary" Cepheids that are in fact members of a triple system and find that at least 44% are triples. The summary of information on Cepheids with orbits makes it likely that the fraction is underestimated.
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Evans et al. (2005) studied this question.
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