Seismology of triple-mode classical Cepheids of the Large Magellanic Cloud

We interpret the three periods detected in OGLE LMC Cepheids SC3-360128 and SC5-338399 as corresponding to the first three overtones of radial pulsations. This interpretation imposes stringent constraints on parameters of the stars and on their evolutionary status, which could only be the first crossing of the instability strip. Evolutionary models reproducing measured periods exist only in a restricted range of metallicities (Z = 0.004-0.007). The models impose an upper limit to the extent of overshooting from the convective core. Absolute magnitude of each star is confined to a narrow interval, which allows us to derive a new estimate of the distance to the LMC. We obtain the value of m - M ranging from 18(.)(m)34 to 18(.)(m)53 with a systematic difference between the two stars of about 0(.)(m)13. The rates of period change predicted by the models are formally in conflict with the derived observational limits, though the uncertainties of measured (P) over dot may have been underestimated. If this discrepancy is confirmed, it would constitute a significant challenge to the stellar evolution theory.