The anomalous Cepheid XZ Ceti

Abstract

School of Physics A28, University of Sydney, NSW 2006, AustraliaReceived / AcceptedABSTRACTAims. XZ Ceti is the only known anomalous Cepheid in the Galactic field. Being the nearest and brightest such variable star,a detailed study of XZ Ceti may shed light on the behaviour of anomalous Cepheids whose representatives have been mostlydetected in external galaxies.Methods. CCD photometric and radial velocity observations have been obtained. The actual period and amplitude of pulsationwere determined by Fourier analysis. The long time scale behaviour of the pulsation period was studied by the method of theO −C diagram using the archival Harvard photographic plates and published photometric data.Results. XZ Ceti differs from the ordinary classical Cepheids in several respects. Its most peculiar feature is cycle-to-cyclevariability of the light curve. The radial velocity phase curve is not stable either. The pulsation period is subjected to strongchanges on various time scales including a very short one. The ratio of amplitudes determined from the photometric and radialvelocity observations indicates that this Cepheid performs an overtone pulsation, in accord with the other known anomalousCepheid in our Galaxy, BL Boo (V19 in the globular cluster NGC 5466).Conclusions. Continued observations are necessary to study the deviations from regularity, to determine their time scale, as wellas to confirm binarity of XZ Ceti and to study its role in the observed peculiar behaviour.Key words. Cepheids – Stars: evolution – Stars: individual: XZ Cet – Stars: individual: BL Boo – binaries: spectroscopic