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 an 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 non-zero 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 .