DO THE MAGELLANIC CEPHEIDS POSE A NEW PUZZLE

The observed Magellanic Cloud (MC) Cepheid data pose a new challenge t o both stellar structure and stellar evolution. The Fourier analysis o f the LMC and SMC data indicate that the 2:1 resonance (P-2/P-0 = 0.5) between the fundamental mode of pulsation and the second overtone occ urs with a period P-0 of approximate to 10 days, just as in our Galaxy . The implications of this resonance are difficult to reconcile with l inear radiative Cepheid models computed for the low metallicity of the MC. They also require a large upward shift in the mass-luminosity rel ation (MLR), seemingly larger than is comfortable for evolutionary cal culations. The disagreement worsens if we interpret the structure in t he first overtone Fourier parameters near 3 days as the result of an a lleged resonance with the fourth overtone. In contrast to earlier expe ctations, the beat Cepheid data with P-1/P-0 approximate to 0.7, as we ll as those with P-2/P-1 approximate to 0.8, impose weak constraints, at best, on the MLRs.