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Ultra-long-period Cepheids (ULPCs) are important in distance-scale studiesdue to their potential for determining distance beyond ~100 Mpc. We performed acomprehensive search for ULPCs in M31, a local benchmark to calibrate thedistance ladders. We use data from the Palomar Transient Factory (PTF), whichhas imaged M31 using a 1.2-m telescope equipped with a ~7.26 deg2 field-of-view(FOV) camera, usually with daily sampling, since the beginning of 2010. Thelarge FOV, together with the regular monitoring, enables us to probe ULPCs inthe bulge, disk, and even out to the halo of M31. Using a difference imaginganalysis technique, we found and characterized 3 promising ULPC candidatesbased on their luminosities, amplitudes and Fourier parameters. The meanabsolute magnitude for these 3 ULPC candidates, calibrated with latest M31distance, is M_R=-6.47mag. Two out of the 3 ULPC candidates have been reportedin literature, however their published periods from Magnier et al. are abouthalf of the periods we found in this work. The third ULPC candidate is a newdiscovery. We studied 5 other candidates and determined that they are probablyMira-like or ultra-long-period variables, but not ULPCs.

Using the Palomar Transient Factory to Search for Ultra-Long-Period Cepheid Candidates in M31