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We report the discovery of 45 beat Cepheids in the Large Magellanic Cloud(LMC) using the MACHO Project photometry database. The variables which arepulsating simultaneously in two radial modes are shown to break cleanly intotwo period-ratio groups, providing the first unambiguous evidence that thesecond overtone is indeed excited in real Cepheids. Thirty stars are beating inthe fundamental and first overtone mode (F/1H, with a period ratio in theneighborhood of 0.72), and fifteen stars are beating in the first and secondovertone (1H/2H, with a period ratio near 0.80). The F/1H period ratios aresystematically higher than known Galactic beat Cepheids, indicating ametallicity dependence whose sense is in agreement with theory. Beat Cepheidsin the LMC are found to select the 1H/2H mode for fundamental periods shorterthan 1.25 days. We find the fraction of Cepheids excited in two modes to beabout 20\% for stars with fundamental periods shorter than 2.5 days. We fail toconfirm any of the proposed beat Cepheid candidates common to our sample fromthe surveys of Andreasen (1987) and Andreasen \& Petersen (1987). We alsopresent finder charts and find several of the beat Cepheids to be in or nearLMC clusters.

The MACHO project LMC variable star inventory. 1: Beat Cepheids-conclusive evidence for the excitation of the second overtone in classical Cepheids