BL Boötis

classical Cepheids and the type II Cepheids

eclipsing binary. It was subsequently thought to be an RR Lyrae variable by T.I. Gryzunova in 1971.^[7]

K lines and A9 or F0 on the basis of its hydrogen lines. The discrepancy is due to a strong deficiency in metals, around 100 times lower than that of the Sun. Despite the spectral classification, it is not thought to be a main-sequence star, at least not a normal one. It is larger and more luminous than its spectrum would indicate, comparable to a horizontal branch star but more massive than comparable stars on the horizontal branch.^[4] The pulsations are thought to be in the first overtone.^[5]

anomalous Cepheid or BL Boötis variable.^[9] These stars are somewhat similar to Cepheid variables, but they do not have the same relationship between their period and luminosity. Their periods are similar to the ab subtypes of RR Lyrae variables; however, they are far brighter than these stars. Anomalous Cepheids are metal poor and have masses not much larger than the Sun's, on average, 1.5 solar masses.^[9] The origin of these stars is uncertain, but thought to possibly be from the merger of two stars.^[10] Detailed examination of the spectrum of BL Boötis with the Keck-1 telescope at the W. M. Keck Observatory showed that its effective (surface) temperature is around 6450 K at minimum light. It also showed that the chemical composition was consistent with ageing metal-poor (Population II) stars and hence cast doubt on the origin as a result of a stellar merger. The radial velocity is lower than would be expected if it were from a stellar merger.^[6]