Period Change Rates in Large Magellanic Cloud Cepheids Revisited
  The period-change rate (PCR) of pulsating variable stars is a useful probe of
changes in their interior structure, and thus of their evolutionary stages. So
far, the PCRs of Classical Cepheids in the Large Magellanic Cloud (LMC) have
been explored in a limited sample of the total population of these variables.
Here we use a template-based method to build observed minus computed (O-C)
period diagrams, from which we can derive PCRs for these stars by taking
advantage of the long time baseline afforded by the Digital Access to a Sky
Century @ Harvard (DASCH) light curves, combined with additional data from the
Optical Gravitational Lensing Experiment (OGLE), the MAssive Compact Halo
Object (MACHO) project, Gaia's Data Release 2, and in some cases the All-Sky
Automated Survey (ASAS). From an initial sample of 2315 sources, our method
provides an unprecedented sample of 1303 LMC Classical Cepheids with accurate
PCRs, the largest for any single galaxy, including the Milky Way. The derived
PCRs are largely compatible with theoretically expected values, as computed by
our team using the Modules for Experiments in Stellar Astrophysics (MESA) code,
as well as with similar previous computations available in the literature.
Additionally, five long-period (P>50 d) sources display a cyclic behavior in
their O-C diagrams, which is clearly incompatible with evolutionary changes.
Finally, on the basis of their large positive PCR values, two first-crossing
Cepheid candidates are identified.
