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The instability strip (IS) of classical Cepheids has been extensively studiedtheoretically. Comparison of the theoretical IS edges with those obtainedempirically, using the most recent Cepheids catalogs available, can provide uswith insights into the physical processes that determine the position of the ISboundaries. In this study, we investigate the empirical positions of the IS ofthe classical Cepheids in the Large Magellanic Cloud (LMC), considering anyeffect that increases its width, to obtain intrinsic edges that can be comparedwith theoretical models. We use data of classical fundamental-mode (F) andfirst-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog,together with a recent high-resolution reddening map from the literature. Ourfinal sample includes 2058 F and 1387 1O Cepheids. We studied their position onthe Hertzsprung-Russell diagram and determined the IS borders by tracing theedges of the color distribution along the strip. We obtain the blue and rededges of the IS in V- and I-photometric bands, in addition to $\log T_{\rmeff}$ and $\log L$. The results obtained show a break located at the Cepheids'period of about 3 days, which was not reported before. We compare our empiricalborders with theoretical ones published in the literature obtaining a goodagreement for specific parameter sets. The break in the IS borders is mostlikely explained by the depopulation of second and third crossing classicalCepheids in the faint part of the IS, since blue loops of evolutionary tracksin this mass range do not extend blueward enough to cross the IS at the LMCmetallicity. Results from the comparison of our empirical borders withtheoretical ones prove that our empirical IS is a useful tool for constrainingtheoretical models.

Empirical instability strip for classical Cepheids: I. The LMC galaxy