V473 Lyr, a modulated, period-doubled Cepheid, and U TrA, a double-mode Cepheid observed by <i>MOST</i>

Abstract

Space-based photometric measurements first revealed low-amplitude\nirregularities in the pulsations of Cepheid stars, but their origins and how\ncommonly they occur remain uncertain. To investigate this phenomenon, we\npresent MOST space telescope photometry of two Cepheids. V473 Lyrae is a\nsecond-overtone, strongly modulated Cepheid, while U Trianguli Australis is a\nCepheid pulsating simultaneously in the fundamental mode and first overtone.\nThe nearly continuous, high-precision photometry reveals alternations in the\namplitudes of cycles in V473 Lyr, the first case of period doubling detected in\na classical Cepheid. In U TrA, we tentatively identify one peak as the $f_X$ or\n0.61-type mode often seen in conjunction with the first radial overtone in\nCepheids, but given the short length of the data, we cannot rule out that it is\na combination peak instead.\n Ground-based photometry and spectroscopy were obtained to follow two\nmodulation cycles in V473 Lyr and to better specify its physical parameters.\nThe simultaneous data yield the phase lag parameter (the phase difference\nbetween maxima in luminosity and radial velocity) of a second-overtone Cepheid\nfor the first time. We find no evidence for a period change in U TrA or an\nenergy exchange between the fundamental mode and the first overtone during the\nlast 50 years, contrary to earlier indications.\n Period doubling in V473 Lyr provides a strong argument that mode interactions\ndo occur in some Cepheids and we may hypothesise that it could be behind the\namplitude modulation, as recently proposed for Blazhko RR Lyrae stars.\n