Anomalous Cepheids (sometimes called BL Bootis stars) are relatively massive (1-2 solar masses), metal-deficient pulsating stars, which spread between classical and type II Cepheids in the period-luminosity diagram. There are two leading hypotheses for the origin of anomalous Cepheids: that they are intermediate-age stars with exceptionally low metallicity, or that they are coalesced old binary stars. Most of the known anomalous Cepheids were found in nearby dwarf spheroidal galaxies and only a few such stars were identified in globular clusters. The OGLE project discovered as many as 83 anomalous Cepheids in the Large Magellanic Cloud (Soszyński et al. 2008). This is the largest sample of such objects ever detected in one stellar environment. On the other hand, in the Small Magellanic Cloud we found only a few candidates for anomalous Cepheids (Soszyński et al. 2010), despite the fact that this galaxy hosts a higher number of classical Cepheids than its larger neighbor. Below is the period-luminosity diagram for Cepheids in the Large Magellanic Cloud. Red and orange points show the position of anomalous Cepheids pulsating in the fundamental mode and first overtone, respectively.
Period-luminosity relations of classical (grey points), type II (larger grey points) and anomalous Cepheids (red and orange points) in the Large Magellanic Cloud. As the "luminosity" we used the reddening-free Wesenheit index, defined as WI = I - 1.55(V - I) - DM, where I and V are mean luminosities of Cepheids in these passbands, and DM = 18.5 mag is the distance modulus of the Large Magellanic Cloud.
Anomalous Cepheids pulsating in the fundamental mode have periods from a few hours to over 2 days. They have asymmetric light curves with a rapid rise to maximum and a slow decline. Many of the fundamental-mode pulsators exhibit a small bump just before the rise to maximum light. The light curves of anomalous Cepheids are very similar to the light curves of classical Cepheids (for periods above 1 day) and RR Lyrae stars (below 1 day), and the distinction between these types of pulsating stars is very difficult, if not impossible, when objects are at different distances and there is no information about their absolute magnitudes. Below we present OGLE-IV I-band light curves of four anomalous Cepheids from the Large Magellanic Cloud. Move the mouse pointer over the image to see the designation and J2000 equatorial coordinates of a given star.
First-overtone anomalous Cepheids have periods from about 0.3 days to slightly over 1 day. Generally, they have smoother light curves than the fundamental-mode pulsators, with rounded maxima and minima, although some stars exhibit a somewhat sharper maximum.