2009ApJ...695..874B

We examine the properties of 18 long period (80-210 days) and very luminous (median absolute magnitude of M_I= -7.86 and M_V= -6.97) Cepheids to see if they can serve as a useful distance indicator. We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow period-luminosity (PL) relation, so in fact they are more "standard candle" like than classical Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL relation is consistent with zero. The scatter of our sample about the W_I PL relation is 0.23 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a nonzero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids' main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.